Iskandar: NP battery and crit buff on Tactics + increase the numbers a bit. He needs other stuff (something defensive...) to be truly good but part of me just wants to go all in on ATTACK MODE which his current kit is. NP battery and a crit buff are all he's really missing on that front. Maybe tack the Crit buff to Charisma instead.
Lancer Diarmuid: Oh god where to begin. Fix skill 3 because it's a disaster. A quick buff at minimum, but in happy ideal world, give him something like Musashi's first skill. His biggest issue right now is he's a quick servant whose hitcounts suck, he could really use it, and he conveniently dual wields which is what the skill is meant to represent. Besides that... Love spot. Make it do something else, a Defense debuff so he can use it as a steroid maybe. A NP damage down to turn it into a mini Nightingale NP vs females. Idk. Something.
Alexander: up Fair Youth success rate at level 10 to 100% and throw something else on it. I dunno what. Star bomb because his cuteness makes him sparkle?
Astolfo: Quick buff on Monsterous strength so that it's less underwhelming, though honestly like Diarmuid what he really needs is improved hitcounts. Unlike Diarmuid, I can't think of a justification for a Musashi's first skill status. Maybe give him like a stupidly huge stargen/np gen buff such that he's getting way more out of the few hits he does get. It'd be kinda whack to do that though
Jeanne: Please make True Name Discernment not bad. Knowing a true name is a much bigger advantage than a tiny NP strength down.
Amakusa: Buff Revelation with, uh, something. Crit up maybe? idk. I like this guy and have him on both my accounts.
Iri: Any of the following: targetted or teamwide debuff clear/immune, np gain up for herself, or teamwide def buff. Shiki has officially opened the floodgates of welfares being buffed. Let's give it to welfares that like, legitimately need it. This poor girl's been like, totally outclassed by snakeboi. It's fine to keep her defensive, but she could use more than just the heals.
Kiritsugu: Part of me wants to say "buff magecraft to give him class advantage vs Casters." This is nice and lore accurate but... it doesn't actually address any of his actual problems. So I'd use his time magic to justify giving him a variant of Musashi's first skill that only works for Art cards. Double (maybe triple I don't feel like doing the math to figure out which would be more balanced) arts hitcount for 3 turns. Not broken because his base NP gain is trash and it doesn't make his Busters/quicks completely insane. Just makes his arts not garbage.
TLDR: I love Musashi's first skill.