Back on topic, I think it's entirely possible we'll have 8 Pokémon, especially if we only get one newcomer. The 6 from Brawl, Greninja, Mewtwo. Mewtwo would almost feel like a newcomer. I know it seems like a lot, but people forget that as the roster expands, more obscure characters are allowed. Plus, Mario also has 8, if you count Yoshi, Wario and Donkey Kong.
Squirtle/Ivysaur: +Starters in XY, Mega Evolutions
-Mega Evo. wouldn't make much sense*, existed in Brawl for Pokémon Trainer
*There is an item in the card game called the devolution spray, so it's not entirely impossible.
Mewtwo: +TWO Mega Evolutions, in Brawl's code
-Wasn't in Brawl, might be too similar to Lucario+Greninja