Updating the classic characters is always an interesting topic, but you never what you'll really get, since in each game, only a few get such drastic changes, like ZSS, Bowser or Zelda. Some receive smaller changes like DK.
I kind of doubt Mario will have his Side-B changed, since he's had it for so long, and it is pretty useful. I know I'm not the only one who would be okay with F.L.U.D.D being changed. Cappy would make sense if it just hit stuff, I'm pretty sure some enemies can't get captured, so it wouldn't be too weird. Though the version of that attack that circles around Mario would be more unique than a straight projectile. The Ground Pound is also a move he's had since Mario 64, and even made into some 2D games with NSMB. But Yoshi and Bowser have their own versions of it.
I don't know what I'd change for Peach. I would say her neutral B I guess, since I find it has the least amount of use the few times I do use the character, there's so many other counters that work better. Though I guess I'd want less counters overall. Giving her a fireball like Mario and Luigi would make sense, but be kind of generic.
She did get her own game in Super Princess Peach. If you wanted to give her an attack from that, either the projectile from the umbrella or have her pick people up with it, again, like in that game.