Character that are a shoe-in:
Lil Mac: Punch-Out!! is an important classic for Nintendo, and with Punch-Out!! Wii being a success, Lil Mac is as relevant and important as he was before. Unlike other rather demanded characters in the internet like K.Rool, he'd be more recognized by the media.
Additionally, he's a boxer, a diverse martial arts. You can go crazy with a character like Lil Mac, and since he closely resembles Mike Tyson in terms of boxing style (he's an in-fighter), it wouldn't be out of character to give him the peek-a-boo stance and devastating hooks, straights body blows, crosses and especially a monsterous right uppercut. (Which is Lil Mac's and Tyson's best moves.) They can think outside the box and give him something like a corkscrew punch, or even a flicker stance to give him more reach.
He'd also be a realistic martial artist, which Smash lacks, and a realistic character in general. (As of now, Snake is the only one, but even he isn't guaranteed to stay.)
Personally I would love for him to be in, since I'm a huge boxing fan. Seeing a character based on a favorite sports (next to baseball) or martial arts is a great thing for me.
Characters that could be in:
Ridley: An important and iconic character to the Metroid series. Being the main villain, he'd obviously be the best choice for another Metroid representative. Sadly enough, his inclusion would be difficult, considering his abilities and overall body structure. I want him in because I like Metroid, and Ridley's actually a favorite of mine.
F-Zero rep: Although Samurai Goroh is the most likely, I want a character to be based on his or her own vehicle. I would love to see Black Shadow in, while having Black Bull translated into his moveset. (Like how the arwing was translated into Fox and Falco.) Personally though, I'd want Pico, Mighty Gazelle, Bio Rex or Deathborn.
Captain Syrup: A Wario Land rep, a pirate and a female all at once. She's a perfect choice for another Wario rep, and with Wario Land: Shake It! she isn't entirely irrelevant anymore
Characters that probably will not be in ever:
Karate Joe: Like why not? . He'd be another martial artist. They can give him full-fledged Karate moveset and still be able to retain the wackiness and charm of the Rhythm Tengoku/Heaven/Paradise series. To add to that, he's iconic to the series, being in all three games, so he's not really too far-fetched. The other characterss from the series I can see being in is
Samurai, who also has been in all three games.
But if the series is to be represented in someway (and it most likely will), it'd be a stage with a lot of music from the games.
Warioware rep: Kat & Ana is an easy choice. Though I really doubt we'll see any Warioware character outside of Wario him.
Now for characters that I want to STAY who got a chance to leave are Snake and Toon Link. Snake's a favorite of mine (with Wario) and Toon Link is really the only iteration of Link I like.