It's happened, Pokémon X & Y has given Mega evolutions to the Kanto starters.
According to the Official Nintendo Magazine, you'll get one of the Kanto starters from the game's Professor, while the X & Y starters (Chespin, Fennekin and Froakie) will be obtained from your friend. This means you get to start your latest Pokémon adventure with two starters. Pretty cool stuff.
ONM also revealed how Mega evolutions actually work. For a Pokémon to reach this level, their trainer needs the Mega Ring and Key Stone items. The Key Stone will then resonate with a Mega Stone held by a Pokémon, which causes the evolution to trigger.