Feel free to give me your reasoning based on what the Hero's could be, since I have a feeling for each is based on their ultimate class.I'll be going in order of game, so here we go!
Red Bow Alm. Sure, he has Falchion. But it was only really needed to slay Duma, in turn, his legendary could (and really should) be a red bow. I'm unsure as to his skill, but I have a feeling it'll be some super Brave Bow, akin to Double Lion. That and he can use bows in the game so why not?
Celica as a Dagger/Stave. Now, this may be our only Legendary Dagger, and I think she will be. Given that we got super Ragnarok via a refine, it's unlikely we'll get another Ragnarok. As such, I'm going to assume that we're getting a Legendary Healer Via Celica, or a Legendary Dagger via her. With Staff's being the most likely. And why a dagger as a suggestion at all? Well, she had a Golden Dagger in SoV that turns to Beloved Zofia. So thats a maybe.
Seliph as Red Cav. It's his final class in FE4 Gen 2 if I recollect, and he would do well to get a new Tyrfing. Preferably a way to counter Julius for lore reasons, and counter most mages. I'm unsure as to what else I'd give him. Maybe a unique blessing skill (like somebodies blessing or something) that'll make him a beast. He wasn't a god in his game, but he should at least be usable and not utter trash if he's gonna be a protag. Hell, Roy who was terrible in his game is being treated better here skill wise.
Leif, Cav Sword. the Braggi Sword needs it's owner. And unless they make Leif a Infantry Sword, in which case not much will change (Cav Sword since Quan was a Rider in his game). I'd imagine that he'll have some unique skills as well, but I can't really think of any. the Braggi Sword is his namesake, so I really don't know.
Roy, Infantry Sword. Binding Blade gets a super boost, and he gets DC (hopefully) alongside some sick as skills. Like a omni chill to "bind" his enemies. Hopefully not -7 to all, so maybe -4 to enemies that have some lower stat, akin to Aversa's tome. This would make him much better here than his own game (cause he sucks weewee in FE6 stat wise) so it'd be a nice boost to him.
Eliwood, Cav Sword. He's gotta get a newer, more potent version of Durandal. Probably gonna wreck house and be an anti dragon. This would be on par with his FE7 performance and he'd be a beast considering how strong dragons are and will be. Think of a Cav Based Marth with higher atk and less spd. If I had to make a guess.
Eirika: Thunder Sieglinde. To counter Eph's Flaming Siegmund unless theyre only going 1 LH for Sacred Stones since they're doubling down on Eph's path as the main path (all the things Lyon says in his screen are answers/references to Eph's questions/chats with him). I would probably keep her near Marth's statline, but she'd be more damage based. Eph heals with his Solar Brace, so I imagine Lunar Brace will do more damage (akin to adding a % of damage to her specials) so she could potentially have a Black Luna esque skill with Luna attached if they go this route.
Micaiah: Blue Tome or Flying Healer. At this point, after seeing how many frickin Swords we'll get, I'd recommend something along the lines of a super Thani or some Yune based tome for her. That or the Ashunera Staff from Radiant Dawn in which case sure. Either way she'll be good, we have no doubt about that.
Chrom:Infantry Sword. You guessed this one. Akin to what I'd expect from Chrom who's not on a horse as a Great Lord (and he's already on a horse in another Alt). He would more than likely get Falchion since they avoided giving it to Lucina, and I'm sure he'd have a unique special similar to the Fire Emblem Marth has since he walks around with it in FE:A.Also something akin to rightul king, so maybe a Steady Breath mixed with a slaying effect on a skill?
M!Robin with Red Flier Levin "Sword" or Blue Flier with a Levin "Sword", preferably a flier. I'd expect a lightning breath effect with a super rally (rally spectrum like FE:A Was unique) and a special Ignis (Ignis was uniquely his in FE:A as well). as a flier, he would float, akin to how he floats in Hero's (Grima) and Warrior's. It'd mix up our sword list not giving us another infantry sword.Levin Sword as a tome, and make it Blue or Red. (Also in FE:A, and Warriors, Levin Sword aren't actually swords, they're focuses for magic, they're only used to summon lightning, the "Sword" being a tome isn't that farfetched.)
F!Corrin & M!Corrin Sword Infantry / Green/Red Dragon (or a repeat color) Omega Yato is a maybe, and getting a dragon Corrin is a must for the future. I do however doubt she'll be a red dragon or a green dragon, so colorless or blue will be more likely. Probably going to be very fast and offensive considering their stat lines. Maybe our first, really good speedy Dragon?
Xander as Armored Sword. Why you may ask? Well, Ryoma's Legendary Alt is based after his other reclass, a Kinshi Knight that can use swords (not doable in Fates btw). However, Xander has a similar situation. He can reclass to General and has can't use swords. He could potentially use one however.
This is a long read so hey, thanks for making it to the bottom. Hope you enjoyed it!