Interestingly enough, it's gotten to the point where almost every currently existing series only has females or villain/rival-types of characters to offer.
Pretty much:
Mario- Complete and, at this point, more characters is just icing on the cake as long as they don't go overboard or chose anyone spectacularly stupid like a goomba.
DK- Dixie and K.Rool are pretty much the two people left here. The franchise has had waning influence in recent years so, if you ask me, it'll probably be lucky to get a new person.
LoZ- Complete if you ask me. The only major reoccuring character not represented is Tingle.
Yoshi-The Mario and Yoshi are virtually one in the same so more characters to represent the Yoshi aspect would be pointless.
Wario- There's a lot of characters who could further represent the Wario Ware franchise but the bigger questions are who and is it necessary?
Metroid- Ridley's pretty much it. It's either him or no one else if you ask me.
Kirby- Complete. You have the three main players and there really isn't the need to add anyone else.
Pikmin- At this time, you have Louie but, if you ask me, is way too similar to Olimar to make for an interesting character.
Star Fox- I used to be all for another Star Fox character but now I'm not sure. Krystal's pretty much it when it comes to potential new characters. The future of franchise is sketchy. There have been a few third party studios who have come forward and said they'd be interested in making a new Star Fox and Miyamoto acknowledges that fans want a new installment but, as of right now, the franchise is on life support. With that being said, does a potentially dead franchise need four characters?
F-Zero- Hasn't had a game since 2004. Probably dead until further notice.
Fire Emblem- Will no doubt be represented by the newest protagonist of the latest game.
Mother- Dead until further notice. Itoi has walked away but there's always the possibly of someone else eventually picking it back up.
Pokemon- Will probably get someone from the 5th generation. (My bet's on Zoroak because he's slowly shaping up to be this generation's Lucario.)
GnW/ROB/Ice Climber/Kid Icarus- Don't need anybody else.
Third Party- At least one new third party character is to be expected.
If you ask me, SSB4 will probably put more emphasis on introducing more franchises to Smash Bros as well as reaching further for interesting, obscure characters.