I think I should make a list of third parties I've been hearing about, with my opinions on them. Here goes.
Mega Man: Mega Man is popular everywhere. While I haven't played any of his games, I like the character, and I have to thank Super Smash Bros. Fan for making me interested in him. I have also used him in SSF2 and SSBC. He's my most wanted 3rd party.
Pac-Man: The news of Namco-Bandai's involvement in SSB4 suddenly lead to an jump in his fanbase and chances. I've heard arguments both in his favour and against him. I've read a few movesets and the list of his power-ups (while huge, many are incredibly unlikely to appear in his moveset). While I wouldn't mind him being included, I find him boring.
Lloyd: I came to know about him through Super Smash Flash 2. While I'm not sure about his chances, the idea of double swords using fire and ice is interesting. Sadly, my interest in him has slowly decreased over time. I still support him, though not as much as before.
Prof. Layton: I came to know about him through SmashBoards. A gentleman sort of character interests me, and also because he reminds me of Sherlock Holmes. I played his first game (Curious Village) (on a ROM (forgive me, I couldn't find it for sale anywhere.)) I really liked the game, and he is almost on par with Mega Man on my list of wanted 3rd parties.
Tekken characters: I haven't played any Tekken games, as I'm not interested in fighting games in general. So, I'm neutral on their inclusion. Out of the Tekken characters, I'm only slightly interested in Heihachi (he reminds me of a political cartoon which is very popular where I live. lol)
Travis Touchdown: Another of the 3rd parties I didn't know about before joining SmashBoards. After doing some research, I realized he is the only 3rd party who I do NOT want in Smash. EVER.
A 2nd Sonic Rep: As long as it's Tails or Eggman, I'm fine.
A 2nd Metal Gear Rep: Neutral on that, since I'm unfamiliar with the series.
Meat Boy: HELL NO!
Whew, that took a while to type. Again, I remind the reader that this an opinion, so don't take it too seriously.