In today's Pokemon-themed Nintendo Direct, it was announced that not only will players receive the original starters from Red and Blue in Pokemon X & Y—Bulbasaur, Charmander Squirtle—but these Pokemon, once fully raised, will be able to Mega evolve.
Take a look:
Additional info, from the Pokemon website: when mega evolved, Venusaur's ability becomes "Thick Fat," which gives it protection against fire and ice moves.When Charizard mega evolves, its special attack stat grows, and its ability changes to "Drought"—which increases fire-type moves. Finally, when Blastoise mega evolves, it gains the "Mega Launcher" ability, and that makes moves such as water pulse and dark pulse become stronger.
Confused? You should read about mega evolutions here!