...Knowing his luck, it's probably Zuko himself that's wrong.
It's all the little things that prove he's wrong. It's not even his fault this time, it was a reasonable assumption, but... this is definitely not the spirit realm. Beyond that, it seems like he's... in the past? And yet it's real. And he can't see any evidence or motive for it to be a spirit's trick, either, or even an extended hallucination - unless he inherited a lot more of the family madness than the thought. But his imagination could not have conjured all the thousand little details.
It's in the way Azula is eleven again, both in age and bending skill. He even wins that first scrap against her, which would usually be impossible without redirecting lightning at least once. He didn't even need his swords. She gets a facial expression he's never seen on her before, more than just rage - shock, loss, underlying upset sadness, is that almost a tear? - and runs off, then avoids him as long as she can.
It's the way the Fire Palace is so present, every sense confirming its reality. The halls are decorated as intricately as he remembers, and he traces details too fine to be conjured. The kitchens smell like home. His rooms - his rooms as a prince, which he never brought himself to properly return to after his banishment, and which he will again be leaving once the Fire Lord's chambers are cleared for him - his rooms bring up memories he'd thought forgotten, right down to the earrings he'd thought about once, then hidden where Azula would never find them.
They hadn't made it to the Wani with him. By the time he had the chance to reconsider, he'd only had one ear worth piercing, and was constantly consumed with tasks besides. He reconsiders now, and has pair both daringly placed in his right, even getting additional rings to pierce the top of both ears. They remind him this is a different time. Tugging lightly at the metal grounds him, and even when reaches for his left ear instead...
It's in the way he has that ear, almost all of it, having healed far, far better this time around. Bending healing has always surpassed mundane medicine, especially considering he'd once tried to recover on a ship. His golden flames reduce the damage and scarring both, and the whole mess is stably settled in a mere week of treatments. This time, there is no infection, not even much hearing loss, and he can see from his left eye. Not perfectly, but enough that he has to readjust to having natural depth perception after decades without.
Then he has to reconcile, again, that his father had truly crippled him once, and tried to kill him with lightning besides. As a thirteen year old.
It's in the way he feels his age. His body is smaller than he expects, and he spends long mornings regaining the effortless grace of his katas, both dao and bending. Adults around him get a look to them, which he starts to recognize on instinct, an irritating mix of damn this bratty upstart and he thinks he's so grown up how cute. It makes him feel small and young, and he learns to hate it. (Youth was not a good time for him, the first time).
In other ways, he feels old. No one understands his new bending forms, even though he taught them publically for years after meeting the dragons. No one recognizes what he references, and especially not his occasional joke. He feels like Aang must have once, thrown out of his time into an unfamiliar land, in drastic circumstances.
It's in the way there is still a war. This is the first thing he changes, even before the reality sinks in. He is numb through his coronation, (barely noticing Azula seated to the side, fists clenched), but he follows the ceremony. The Fire Lord title grants him power. The way the Fire Nation is still poised, expecting decades more of war, in favorable positions, gives him power. There is no avatar and little hope among the other nations.
He uses this ruthlessly and with little shame.
The fresh regiment that started the whole mess of an Agni Kai is never deployed.
It's in the way the pain of his father's death is fresh, and mixed with a boundless well of fresh horror. No matter how he avoids the thoughts, they invade his nightmares without fail. He killed the man, this time. With his own lightning, sure, but it had hurt when his other father passed in his sleep decades ago. Now, all the unresolved feelings he'd once packed up like a wound are ripped open again, and bleeding besides. He'd never quite managed to stop loving his blood father, no matter that Uncle took up all the responsibilities that mattered.
It's in the way his Iroh is there, yet acts so slightly different than Zuko once knew. His presence is invaluable, a grounding force, always available for a hug when his warm arms are the only thing holding the pieces of Zuko from shattering apart. He even appreciates the damn proverbs and tea this time, and submits himself to playing pai sho, though still never the tsungi horn.
But there is a barrier, now, that Zuko begrudgingly recognizes from his teenage years travelling with the man. Iroh is expecting a hurt teenager, and so that is what he sees, and whom he speaks to. The long, political conversations they'd grown into - once spending a whole evening debating the economic value of different regions' strains of rice - have become short, one-sided, with Iroh breaking away into pedantic philosophy before ever truly engaging. Worse yet, he catches his uncle looking at him sometimes, with something he's never seen before and can't (refuses to) name. He realizes what it is, after Iroh initiates some probing questions, when he sees the man's face at his father's funeral. Zuko killed a man at thirteen. His uncle is not living one thin ship's wall away to overhear the nightmares it causes him, and his court face has suddenly grown flawless.
It's in the way he is feared. The Agni Kai was only ever going to end one way - a dead crown prince. Instead, his people watched him go from begging forgiveness and accepting a burn one moment, to returning lightning to his own father's heart. They forget how the deadly lightning came from Ozai himself, and instead remember how the young prince bowed flawlessly as Fire Lord only instants after killing the previous. They see his well-practiced court masks, the way his burn (lessened or not) leaves a permanent glare on his face, and see none of the decades making it a necessary practice.
The other nations do not associate him with peace. He is not the Avatar's teacher, valiantly helping to overthrow tyranny and usher in balance and harmony. He is the upstart who wrest the throne from his father too soon, then decided to end a hundred-year war on a whim. Never mind that he has never explicitly or intentionally threatened them; never mind that he brokers a far fairer deal than his position requires. He is the unilateral leader of a nation poised to finish conquering the world, and no one ever quite forgets that.
It's in the crushing loneliness of realizing his friends are gone, and his family not truly returned. Mai and Ty Lee come to visit Azula, and greet him formally as Fire Lord. He can see in the way their eyes flicker, the way they tuck themselves behind his sister, that their main concern is whether he will use his new power to retaliate for the times they joined Azula in pestering him. He stays by his turtleduck pond for hours.
Even if he was able to seek out old friends without his position hanging in between, his once-friends are back as children. The arrival of the Southern Water Tribe delegation drives this home, in that Sokka and Katara are not even present - too young to be brought along, even for learning - and the Chief looks like a younger version of the other Sokka he remembers. Resemblance or no, Hakuda is not his son, and worse, gives that look-down-the-nose that Zuko recently learned to hate. They are still, if barely, able to speak with a veneer of respect, and broker treaties.
In this way, it is all worthwhile. The Southern Raiders are withdrawn, and half the Water Tribe has never lost is benders (none are imprisoned, now, and Zuko prays to La that bloodbending will never come to be). Ba Sing Se stands proud, having repelled the only Fire Nation royal to threaten its walls (only months after Zuko has brokered the other nations' peace do they deign to send a delegation, with yet longer before King Kuei is aware enough to be part of it, and even then only at the Fire Lord's insistence). Yue, whom he recognizes from stories, arrives with the Northern delegation, which must mean the moon stands stronger than ever. A fleet of fire navy ships - ships full of people, if not innocent ones, then at least his people - will never be drowned by a raging ocean spirit. Another fleet's worth will never die repelling an invasion, or falling with their broken airships.
The mediation courts are formed, in a much more thought-out structure than his first fragile attempt. He advocates peaceful resolutions fiercely, and gets far too many knowing looks. They are wrong in their knowing, but at least it helps with the fear. Between this, and the part where he is a spirits-damned teenager, fewer war hawks are able to force him into a full-out Agni Kai.
Beyond annulling deaths, he is able to save lives. Bans are once again lifted from dancing and cultural acts. This takes extra care in the colonies - both former and remaining - to deliberately reintroduce the silenced practices of their suppressed nationality.
Zuko carefully acquires and dedicates a region to support all the nations' cultures, selected in a location to support all their elements. He reassembles practices for a council among all the nations, to perform much like the new courts in preventing another act like Sozin's.
The first meetings are spent sniping about the way he, and he alone, was the initiating force to their creation, and thus held disproportionate power in what should be equal standings. Drawing on decades of political power plays, Zuko politely tells them off, reminding them that he only has more power because they believe so and allow it to him. The next several meetings see them blatantly testing this, once going so far as to suggest the absurd notion of a council with everyone but the Fire Nation. Zuko patiently stocks his office with headache remedies, reminding himself that these are the same delegates he is accustomed to. Often they are literally the same person, albeit younger, and this leaves them less trained, while the young-old Fire Lord has already learned all their tricks. This allows him to subtly manipulate countless meetings to his own ends, pushing toward equity and integration. Iroh, of all the delegates, is the first to notice that Zuko is not only keeping the Fire Nation in good standing, but deliberately pushing the other nations to grow for a more powerful balance.
Uncle takes him aside, then, and over a game of pai sho offers a brand new lotus tile and an explanation. Zuko remember how this discovery went in his past life, and puts on a smirk. He takes the tile, and in a petty but vindicating move, he spouts several of the secret phrases his uncle once taught him, including one known only by the Grand Lotus and his most trusted circle. He lies that he learned them from his mother, and if Iroh sees through it, for once he is the one left to wonder. At the very least, Zuko does assure his uncle that their goals align.
Aang awakens to a world no longer desperate for a savior. Knowing the timeline has inevitably changed, and remembering the coincidence of the original story, Zuko places a diplomatic discussion in the appropriate time and area. Any skepticism of the odd meeting is quickly forgotten in the excitement of the airbender's sheer existence, and even once it dies down only Uncle remembers well enough - and believes enough in spirit-tales - to offer a raised eyebrow. Aang is offered lodging in the all-nations region, where he swiftly outs himself as the Avatar, and is subsequently offered a place in either the Airbending delegation to the grand council, or a neutral one. Zuko himself is careful to lower the pressure to take such a position, having known a young Aang for years, not to mention his own experience with the constant demands as an underage leader (twice over).
The airbender accepts anyway, wanting to do right by his role. The Fire Lord takes care to support him, and Zuko find himself befriending the child version of his once-student. Even as an adult, Aang didn't lose much of his childish streak, making the new relationship more comfortable than expected. This gives him the courage to seek out his other former friends, starting with Team Avatar (and as appalled as ever with himself for actually using the ridiculously childish title).
Perhaps less subtly than he intends, he convinces Hakoda to allow his heirs to meet the young Lord. Katara meets him with at least less hostility than the last time he introduced himself as an ally, though still a healthy dose of suspicion. Zuko distracts her by asking about her bending training, and she rapidly descends into excited gestures, watery demonstrations included. He makes jokes with Sokka, duly shattering any barriers there. It feels manipulative, knowing his former friends' interests and weaknesses, but he convinces himself it's worth it to gain their smiles and budding friendship. They begin visiting each other any time the Fire and Southern Water delegations meet, and exchange letters the rest of the time. He introduces them to Aang, who develops the predictable teenage crush on Katara. While he eventually manages, bringing Toph into the group takes more convincing - mostly on her parent's part.
He creates contracts with the Bei Fong merchants, and 'accidentally' meets the hidden daughter of the house in a courtyard. They strike off better than Toph's parents have ever seen. Once, the Bei Fong patriarch offers the Fire Lord his daughter's hand in marriage, which leaves Zuko flustered and sours their relationship for a time. Remembering this issue, Zuko spends time to reassure Toph he knows she's not helpless, which seems to sink in when she feels him in the stands of an Earth Rumble. Together, they discover that he can emulate her earth-sense with the heat around him, though to a much less accurate degree. He'd never considered this last time, and Zuko makes doubly sure to prod her toward metalbending.
Like the other Toph, she's never afraid to sock him playfully. She trusts him, too, perhaps even more than Uncle, since she can verify his heartbeat. She's still able to call him out on his lies, no matter how much better (how much practice) he's gotten.
Because of this, he inevitably slips. Beyond the old memory of her other self, Toph is comfortable to spend time with. Zuko lets down his guard farther than he realizes, losing filters he'd forgotten to need. She catches him in an impossible truth, rather than a lie. Denying it makes it worse; she sets in with all the stubbornness of the World's Greatest Earthbender. The firebender makes a decent showing, but loses the out-stubborning contest.
Toph is the first person Zuko tells about his not-future.
She listens. She feels his heart cry truth. She boggles at him incredulously, she makes him lie to double-check her senses. She yells at him for hiding secrets, stomps away, avoids him. She comes back. She believes him.
She pats him on the head and tells him he did a good job making things better this time around. It means everything to hear those words, to know someone else believes he's worked for good, not just his own selfish wishes, to hear he hasn't made things worse. He breaks down on her shoulder and cries grateful, happy tears for the first time since he realized he had a second chance.
It's been a better one, so far.