eight characters seem reasonable...it would make it 9 new ones if Lucario is included.
1) Ridley: for reasons, there's a thread about it somewhere
2) King K. Rool: I think rather than another monkey, DK series could use the always-tricky K. Rool
3) Krystal: No one in the star fox series can offer a more different gameplay, since Krystal is pretty much the only one not to be originally a pilot. Plus, the staff would make a unique fighting style.
4) Animal Crosser: seems likely, and I wouldn't mind it.
5) Sukapon: this is one of my most random favorites. It'd be like Brawl's G&W.
6) Third Party Character: Megaman would make sense, but Sora would be neat. There's also bomberman, pacman, etc. Oh, and I count Geno in here too.
7) Bowser Jr: gut feeling.
8) Hector: I love Fire Emblem, and there'd be nothing more awesome than having Hector deliver massive pain with his wolfbeil axe (or better, Armads). I don't see him getting the spot, but I'd love it if he did.
I think all those are likely except Sukapon and Hector...those I put in 'cause to me they're awesome.