Returning:
Pikachu- Obviously.
Jigglypuff- I believe they could just port her brawl data over. Graphically I don't think they'll update her (graphically... I doubt ssb4 will be all that more high def...so unless there is some new system/battle damage I have a feeling there will be some ported over characters), moveset... they'll probably keep the same.
Pokemon trainer- Doubt they'll replace. Popular pokemon, and with a decent variety. Charizard being a game mascot and still the only starter that can fly. Ivysaur... well most of the grass starters walk on four legs and are kinda similar....Chikorita line, turtwig line...yeah. Squirtle- one of the three original starters.
Lucario- To me the most likely to get axed/replaced but I doubt it.
Mewtwo- Returns. Popular, and supposedly was in development for brawl.
Possible new reps:
Plusle&minum- Depending on if they were actually in development... and if so how far they were in development I think decides their inclusion.
New pokemon trainer- Don't really see happening. Unless a new mechanic is introduced. Even so don't think they'll do another starter trio. Would be neat to see every gen represented, but would get a bit repetitive.
Zoroark- The most possible out of gen 5. Have only seen bits of the movie... I don't think ended up being nearly as popular as lucario... but who knows.
Gen 6 pokemon- Not much to say here, besides that I think it has a higher chance than gen 5 pokemon.
Other pokemon I think have a shot:
Meowth- Rumored to have been considered since the first game. Pokeball pokemon in original, trophy in melee, and back to being a pokeball pokemon in brawl. Still a main character in the anime, recognizable, and plays up the whole cat and mouse rivalry with pikachu.
Eevee- I can see happening, arguably the most popular pokemon not in the game. Sells a crap ton of merchandise in japan along with its evolutions. Whenever A new game comes out, everyone is speculating on new eevee evolutions.
How it could work: Same kind of set up as Pokemon trainer, with eevee, jolteon, flareon, vaporeon. Possible dlc, or future smash games they could include further evolutions.
Other option: Like reds eevee in the pokemon adventures manga it could change into vaporeon, jolteon, or flareon on will due to team rocket experimenting on it.