So as many of you know, some people have the game now due to Nintendo's accidental release of it for a few hours. And let me just say this game has exceeded my low expectations, while I'm still disappointed that Huey is the only OC, pretty much everything else was actually decent. I think this game might actually be underrated.
Bowser and Luigi both talk, and the final boss is kinda Bowser but kinda not. He's not in his right mind and seems to be controlled by the black paint. This is evident when you drain the black paint from his face and he then snaps out of it for a few seconds to ask why Mario is there. Then the black paint regains control and makes him say "Ignore what I just said." Also Luigi doesn't have a big role till the last stage, where he takes you there on Rainbow Road in a kart... no joke.
Huey sacrifices himself, as expected, but in a much sadder way than Kersti. And he doesn't come back until you 100% the game, in which case he falls from the sky, seemingly lifeless.