[Any other Megaman Battle Network & Starforce fans out there?]

Yeah, I loved these back in the day. They were so much fun...

Even if I always got destroyed by Bass.

(Outside of 2 and 3, where he has a story presence, but then 3's bonus Bass wrecked me like usual)

Even Mega Man Network Transmission (the Battle Network Gamecube platformer) is quite enjoyable (might just be me being a sucker for platformers tho), though sadly I never finished it.

Though I did kinda get annoyed by 4, it just wasn't as good as 2 or 3 (or 1, for that matter, but 2 and 3 were leaps and bounds above 1 and 4 from what I remember). 5 and 6 were fun again tho, so it's all good.

Never touched Star Force, though honestly it didn't look bad, I just never found it.

To be honest, despite the total deviation from how Mega Man normally is, I'd say Battle Network is my favorite Mega Man series (it's close, though, I really enjoyed the X (minus 7) and Zero (minus 4, which I have yet to play) games too).

[Hayato in Revelation]

it's +20%/+15%/-5%/-10%/-5%/-5%/+20%/-10%. 15% Str extra is a good amount at only 5% Mag in while Tier 1

Really? I wasn't aware of that.

And now that I checked his growths, Hayato'd actually do pretty well as an Oni Savage. Not sure I'd go away from Diviner (though in my case, Rinkah's been a consistently acceptable unit for me; if your Rinkah sucks, you may benefit more from Savage Hayato), but it's actually quite a bit better than I expected (especially Defense-wise, he has almost as much as Rinkah, with a far higher HP growth for some reason).

So, to change my answer to the OP's question, go Oni Savage if you want. You can reclass back to Basara after promotion, grab whatever skills you need, and jump back to Chieftain.

[Hayato in Revelation]

I didn't end up using Hayato, but for this question...

Get to whenever you'd promote him, go either Onmyoji or Basara, get a few levels if you want, then Heart Seal over to Oni Chieftain. There's no need to reclass now, since his strength is crap and Oni Savage doesn't seem to give much in the way of Strength growth (at least, not in my experience), so Oni Savage wouldn't be too useful (unless it gave him a respectable Defense growth? But then, Chieftain'd probably do that too, if not better, so still not necessary in my opinion).

But I could be wrong.

[Dragon vein ideas]

That's actually an interesting thought, though I could see it getting kinda confusing (or redundant, if their effects overlap too much) if there were 10 elements (or would it be 11, since Garon should be able to use them too, right?).

Maybe to simplify, just have... I dunno, 4 elements. One element for each Hoshidan and Nohrian royal, as well as Corrin (who gets the element of your choice) and Azura (who can share one with the other royals).

So, uh, here's a few thoughts on what those Dragon Vein elements would be, what they would do, and who would get them (borrowing and expanding on your thoughts a bit, thanks for those mate):

Fire: It'd burn things, burning forests into wastes, melting ice into more-difficult-to-pass water, and melting rivers into easier-to-pass dry land. Hinoka and Xander could share this.

Water: It'd create floods, filling dry land with difficult-to-pass lakes, expanding already-established bodies of water if possible, maybe also freeze waters into easier-to-pass-over ice, and create healing rains that restore the HP and/or cure the debuffs of all characters (friendly or enemy) in an area (or over the whole map). Azura'd get this, as would Ryoma and Camilla.

Wind: It'd create turbulent areas that fliers are slowed by and create wind-gists that either directly move allies and/or enemies in a direction, or create a longer-lasting wind that halves movement costs for moving in one direction while doubling them for moving in the opposite direction. Takumi shares this with Elise.

Earth: Like your Plant, it'd create forests on plains and wastes, and create plant-based bridges across gaps, but it'd also create healing areas on the ground, as well as creating impassable walls. Leo and Sakura would get this.

As for Corrin, just add an option to the avatar-creation menu that lets you select which DV element he/she'll have.

As for children, they'd inherit their mother's element if the royals intermarried, and would get their royal parent's element if only one parent can use Dragon Veins.

Also, here's an element that I don't think would be great to give the player, but which might be interesting if given to enemies...

Darkness: It causes Fog of War, and also creates areas that either directly damage units who start their turn on them, make units who are standing on them have lowered stats (which would count separately from other debuffs), or would make units who are standing on them take increased damage from attacks.

Also, give some of the Invader-type enemies the ability to use Dragon Veins. Not all of them, maybe just specifically-marked ones, or bosses, or something. Just because it frees things up to have chapters where enemies make your life miserable with DVs without having to contrive a reason for one or more of the royals to fight your party (not that that can't be done, it totally should, but for situations where you can't but still want enemies to use DV on your party, that's something you could do).

Just a handful of thoughts. Not necessarily good ones, but that's what you get from me after midnight, haha.

[Annoying Niggling Nitpicks from a Snarky Nohrian Knight.]

I thought Katanas, Naginatas, Clubs, Yumi, Scrolls, Shuriken and Rods would only be equippable by Hoshidan characters, and Swords, Lances, Axes, Bows, Tomes, Daggers and Staves would be Nohr-character-exclusive (maybe with Corrin being able to use all of them, maybe not).

I was wrong.

This isn't a big deal, but it's kind of disappointing, a little, for some reason I'm not entirely sure of now.

[Best ninja?]

Out of Kaze, Saizo, and Kagero... I dunno, honestly.

*Disclaimer: I'm gonna be talking from personal experience, mostly. Also not bothering with marriage/A+ reclass options, because there are lots. Alsoalso assuming Birthright, because I haven't played Revelation and Kaze is obviously the best on Conquest because the other two don't join your party.

I don't really care for Kagero. Never used her, so I may be wrong about some of the stuff I'm about to write.

I doubt she's bad, honestly. She has a great Strength growth and base (15 is good, right?), and her Speed is... the same as Kaze's when he joined you, so it's not great, but she'll get more in a few levels (hopefully). Her Resistance seems like it could be good too (better than her Defense, at any rate). However, her Defense looks like crap, and it probably won't get much better without Dracoshields or good luck (speaking of which... I dunno where exactly the line is for bad Luck, but Kagero's seems acceptable to me... She isn't Arthur, at any rate), and her HP is worrisome (nothing a Seraph Robe can't fix, but still). More importantly, her Skill growth looks rather bad, and her base is something Saizo starts above and Kaze should probably be above in one or two levels (or at least, that happens with mine...). She's probably not bad (1-2 range (3 with Spy's Shuriken) is a great thing, and she does have great Strength).

On her personal skill... Shuriken Mastery is okay if Kagero gets attacked with a Shuriken/Dagger... Don't remember if she still gets debuffed by Shuriken/Daggers tho, but she probably does. Still, it's a very specific Counter with debuffs, which is nice for when Kagero has to eat a hit.

Regarding her promotion... Master Ninja's Lethality has a Skill-based activation, and Kagero's skill isn't good, so probably not worth it. Shurikenfaire is additional damage, and that's always good if you have room, but Kagero's Strength is high enough that she probably doesn't need even more damage. Mechanist's Golembane... Does it work on Faceless and/or Stoneborn? If it does, it can be semi-worthwhile to deal with those, otherwise it's not needed. Replicate, however, is just awesome, giving you an extra Kagero to work with. So long as you're careful (remember, they share inventory and the same HP), it's very useful to have an extra unit, even if it's just to Pair Up with someone.

As for what you can get out of her reclass option... Diviner would be alright if Kagero had a Magic growth. She doesn't though, so it's not worth it until she promotes. After promotion, Basara has Rend Heaven, which she could probably use, and Quixotic, which would help her if she's having trouble hitting (which she could, her Skill doesn't seem good at all). Onmyoji has nothing worth taking, unless you want Rally Magic for some reason (why you would is beyond me, but like I said, I've never bothered with Kagero).

Overall... Kagero cripples foes with high damage and debuffs... If she hits them. And she'd appreciate it if you kept her away from enemies who can hit back, what with her low HP and bad Defense. Maybe she could take a magical shot or two, her Resistance seems rather good. Ultimately, she's probably usable, but I've never bothered with her, since every iteration of the Super Saizo Bros have ended up as more solid (if not just straight up better) units.

I like Kaze, he's alright. His Speed is solid to begin with and just keeps on growing, his Resistance is good enough to make him a solid mage-killer (especially since those don't tend to have much Defense, so Kaze can hit them fairly hard), and unlike Kagero his Skill is solid, so he'll generally have a solid hit rate. His Strength isn't amazing, but it'll get the job done, the job being wrecking mages and dealing a bit of damage while applying debuffs. His Luck is near Arthur levels (not quite that bad, but it's not good, and it probably won't go too much higher), and his Defense is crap, so please keep him away from physical enemies. Overall, he's a very solid mage-killer and debuff-applier, and can dodge very well, but crumbles even faster than Kagero if physical foes can hit him.

Regarding his personal skill... Miraculous Save is like Miracle; nice when it activates, but hopefully you never end up in a situation where you need it to. Not a bad skill tho.

Regarding his promotion... Master Ninja's Lethality is alright for Kaze, since he'll have respectable Skill, but it's not something he really benefits especially much from, while Shurikenfaire is amazing for Kaze, given his relative weakness. Mechanist's Golembane isn't especially useful unless you're up against a bunch of Stoneborn (I think)/Automata/other Mechanists. Replicate gives you two Kazes. No need to elaborate there, that's just a great thing to have.

As for what you can get out of his reclass option... Samurai brings Duelist's Blow, which is awesome for Kaze since he really, really would appreciate extra Avoid, even if it's just while attacking, and Vantage, which is much appreciated by someone as frail as Kaze. Swordmaster gives him Astra, which is a nice plus when it activates, and Swordfaire, which is much appreciated when he's a Master Ninja/Swordmaster/Master of Arms. Speaking of which, Master of Arms brings Seal Strength, which is just amazing for Kaze, since he'd really like to deal with low-Strength enemies if he has to deal with physical enemies, and Life or Death, which... Only if you're really confident in Kaze's Avoid, because he'll be taking far more damage if it fails him.

Overall... Kaze is a stellar mage-mauler and debuff-applier, and he could even serve as a decent avoid-tank in a pinch (don't rely on it too much, but still, you can), but his Defense will always be meh at best, his damage will be low without Shuriken-/Swordfaire, and he's going to die if a strong physical unit breathes on him. He's worth building an A support quickly with.

Saizo is just solid. His HP is alright, his Strength is good and gets better quickly (and his Magic growth is surprisingly high too, so maybe toss him a magic weapon or two), and his Skill is very, very good (the best of the three ninja), his Speed isn't quite Kaze fast, but it's also rather good, and his Luck is solid, and unlike Kaze and Kagero, his Defense starts good and stays there. His Resistance, however, is utter crap, and it's not going to get better without... whatever the Resistance-growing stat booster is (I forgot, sorry). Overall, Saizo's stats are great, so long as you don't let a mage near him.

Regarding his personal skill... Pyrotechnics would be a beautiful thing if it didn't also harm Saizo... and also if it didn't require getting him to low HP to use in the first place. Still, you can throw a low-health Saizo into a group of enemies, chip them down with Pyrotechnics, and heal Saizo afterward, or Rescue him back to safety.

Regarding his promotion... Master Ninja's Lethality is actually probably usable on Saizo, since his Skill is great, but it's not strictly necessary, and Shurikenfaire is alright if you feel his damage needs a boost, but isn't needed either. Mechanist's Golembane is situational, so I wouldn't recommend it. Replicate gives you another Saizo. Why wouldn't you want another Saizo?

As for what you can get out of his reclass option... Samurai's Duelist's Blow is alright. Saizo doesn't appreciate it as much as Kaze does, but it's still a large Avoid boost, and that's nice. Vantage is always welcome. Swordmaster's Astra is pretty okay, it's a bunch of hits, and Swordfaire is more damage, which is alright. Master of Arms's Seal Strength is very nice, Saizo's Defense means that he can push physical damage very low with that as well as helping squishier teammates by lowering enemy Strength. Life or Death... I dunno, depends on whether or not Saizo can take the additional damage. I wouldn't use it, but that might just be me being overly cautious.

Overall... Saizo is great. He can deal solid damage, his Skill means you can rely on him to hit things, his Speed is good, and his Defense means he can function against physical foes (between that and dodging, he can probably last a while). Don't let magical enemies near him though, he and his low Resistance don't want anything to do with them unless he's one-shotting them.

So, to answer your question (based solely on my opinion):

1) Saizo, because he's just really solid.

2) Kaze, for being a great mage-killer.

3) Kagero, for coming later than Kaze and Saizo and being very fragile.

But that's just my opinion. Could be totally wrong.

Edit: holy crap I missed a whole bunch of responses while I was typing this up.