Hoenn already offered the Johto starters late in-game, so instead of adding the Sinnoh starters and giving them Mega evolutions, they could just give the Johto starters earlier in the story line along with their Mega stones, if they were to get Mega evolutions of course. Even that, I think, is rather improbable. After Camerupt and Sharpedo, I don't see many more Pokemon getting a Mega in ORAS - possibly Metagross/Milotic/Flygon, but that would be it. d: