First off, no to every hero having a playable villain to join them, since Smash isn't about the heroes going against their worst enemies from their own games. Not to mention I don't think it would be very original and would take up a lot of character slots that could be used for much better characters, villains or otherwise.
Bowser Jr.: Even if a stage from Super Mario Sunshine is in the game which also happens to be the first game Bowser Jr. shown up in, plus playing a big role in New Super Mario Bros., I still don't really support him. For one, he just doesn't seem different enough from Bowser to me. Now before the paintbrush gets brought up, what more would he do then paint some goon and maybe transform into Shadow Mario? Sure he could have pretty different moves from Bowser, but otherwise, is he really that much different from him? Before someone tries to turn this argument against me with Mario and Luigi, there's quite a difference. Luigi was always somewhat similar to Mario, plus Luigi is a very big icon to the Mario series, much more so then Bowser Jr.
Ganondorf: Yes to the G-Dorf as I see no reason for him to not return. Not really much else for me to say about him.
Dark Link: Other then an special color for Link (like in Melee, except this time not just purely black), no, just no.
Zant: Maybe appearing as a NPC on a Zelda stage or as an AT, because otherwise I don't really support him, at least not over Midna.
King Dedede: I think he's the only other Kirby character besides the pink puffball himself and Meta Knight that could get in. His hammer alone would lead to an original move set and at the same time balance out the numerous sword wielders in the game.
Dark Samus: As I mention in the topic about which females should appear in Brawl, I don't support Dark Samus except as an alternate look to Samus since otherwise she doesn't seem different enough. A dark version of the main heroine doesn't seem great enough to get in over someone like Ridley, who I'm about to get to.
Ridley: The only other one besides Ganondorf that I support getting in Brawl so far. Being a dragon and a trademark villain of the Metroid series, I think he's pretty likely to show up as playable. They could easily give him an original move set, plus he adds a darker mood to kinda balance out the lightheaded side of the game.
Black Shadow: What does he have going for him besides being a villain that shown up in the later F-Zero games? After a certain F-Zero character got denied as being playable, I think putting him in as playable would be a slap in the face to the fans of a certain someone else. I say no to Batman Black Shadow.
Wolf: I'll support him as a 3rd Star Fox character, if Krystal doesn't get in. I just feel like four playable SF characters would be one too many. I'm not really so sure would he could do, but Wolf supporters seem to have that figured out, when he isn't treated as a Falco clone that is. If all else fails, an alternate look for Fox due to their very similar size and shape, not so sure about style though.
Ya, I do come off pretty harsh for most of the villains, but I feel like every character added in Brawl should really count and bring originality to the game play as well as fan service, not getting in solely for being popular with limited originality to back it up.