Ridley: I'm with LD: Samus works alone and Ridley is the only worthy enemy from Metroid. For now, Ridley is a boss.
Animal Crosser: Yes
Dropped Melee characters: Download please
Geno: I picture him more like a territorial, grumpy Gandhi than a fierce, aggressive warrior. No.
Krystal: Of all the Star Fox characters, I must say she's the one that gives me the creeps most.
Waluigi: No no no no no no no no no no no no.
Dixie: Partially programmed in Brawl, who knows
Bowser Jr.: No
Tingle: Get serious
Petey Piranha: ...
Baby Mario Bros./Plusle & Minun/Burt & Ernie: Do you really want them?
Conclusion: Sakurai has nearly perfected the roster, as very few characters left to be suggested are any good.