Charizard, Venusaur and Blastoise, will play a large role in the new game, and have all been given new Mega Evolution forms.
No matter how many Pokemon games they make, the undisputed fact is that the original 3 starters, Charmander, Squirtle and Bulbasaur, are still the best. Eclipsed only by that little yellow rat in popularity, these Pokemon are so beloved that Game Freak have decided that the new Professor in Pokemon X and Y will offer you a choice between the original starters from Red and Blue, rather than the game's new starters as is the tradition. In order to make these special Pokemon even more special, they will each receive one of the new "Mega Evolutions".
Say hello to Mega Charizard, Mega Venusaur and Mega Blastoise, and man do they look awesome. It was revealed that Mega Evolutions occurs when the trainer's bracelet, called a Mega ring, connects with the Mega stone a Pokemon holds. Mega Evolutions are related to a trainer's "bond" with their Pokemon, so we can probably assume increasing your Pokemon's happiness will be essential to unlocking it's Mega Evolution.
The Mega versions of the Kanto starters join Mega Blaziken, Mega Absol, Mega Mawile, Mega Mewtwo, Mega Lucario and Mega Ampharos in the growing list of confirmed Mega Evoltions. Each evolution will require its own Mega stone, and thus far, only the source of Blaziken's Mega stone (which will be available via a distribution event) is known.
While the new game's professor will be giving you one of the original starters, the player character's best friend will gift you one of the new game's starters, meaning players can double up with their starters.