Paper Mario: Oh, wow... He has a hammer, something Mario doesn't have. That instantly makes him the top candidate for Brawl, right?
Right?
Diddy Kong and Dixie Kong Duo: While we're at it, let's make a Mario and Luigi duo, a Fox and Falco duo, a Pikachu and Pichu duo...
Waluigi: Right, because the influx of Mario characters obviously isn't a sign that unpopular nobody's like Waluigi just aren't feasible.
Daisy: Um... Isn't she like, already in Melee? Jeez, fans of her should be lucky that they got that much; she's not significant enough to have a mostly made up moveset for her.
Insert Drone Here: Other than KoopaTroopa (even then, that's only because he's iconic and technically there IS a singular Koopa character), all drone ideas suck. I shouldn't have to explain why, but if you honestly want to play as somebody like Shy Guy or Metroid over Geno or Ridely, you're beyond help when it comes to character suggestions.
Any Sonic character other than Sonic: I doubt the next two 3rd party slots are going to be wasted on one 3rd party franchise: It's unfair to the other companies who have graciously let the team use their characters and it makes the franchise representation in the game unbalanced. This applies to any 3rd party character, but it seems to happen to Sonic the Hedgehog the most.
95% of the Pokemon in existence: The fact that there's a METAPOD thread of all things going on in this board should speak volumes as to how little foresight most people have here.
Alternate versions of characters which could easily be costumes, but nooo, they have to be individual characters: Seriously, what's wrong with the costume idea?
Most 3rd Party choices: Oh lord, while Snake is a wonderfully unexpected yet awesome addition, I can't help but feel his inclusion is the entire foundation for topics like DANTE FROM DMC or MASTER CHIEF. The sad thing is, this is coming from somebody who feels that Cloud getting into the game is a moderate possibility thanks to a certain somebody composing the game's soundtrack and the entire Geno copyright mess.