For the next SSB game they will most likely add one to two new Sonic characters, one new Mario character, one new StarFox character, replace a Zelda character, replace a Pokemon, as well as new characters from other games.

For the Sonic character most likely the Knuckles or Tails will be added over Shadow or Silver. Plus the attacks for Knuckles and Tails would be more unique in comparison to Sonic's attacks. Tails jumping might be similar to Peaches', while Knuckles could be a medium weight power character.

For the Mario character, Bowser Jr. would throw in an interesting character to play. I have not played enough of the Mario/DK characters to know if another would be a better option, but I am certain they will put a new Mario/DK character in.

For the StarFox character I am guessing they will add in Krystal to the game. This would add another girl to the game plus a staff would be an interesting weapon. They might add Leon Powalski but this is less likely.

They will most likely get rid of Toon Link just because he is only from one game. He game play was fun and most people will miss it from SSB. They will probably make Link and Ganandorf faster and possibly lighter so that they can be more competitive in competitions. Another character will most likely be added to replace Toon Link, if they remove him.

They will probably replace Lucario, and they might bring Mewtwo back instead. Just as likely, for promotion of the game, they will add a pokemon from black or white.

These are just some characters I thick they will bring in.
Banjo Kazooie will be brought in because he originally on a Nintendo platform, and he will have an interesting attack set.
Bomberman and/or Megaman: I do not know if I should add them or not but somebody else made the suggestion and I do not see why not. They have been on Nintendo platforms in the past.
The Mii just because it is for the wii. They will all have the same attack set, but there size will determine there speed and power.