Tsunekazu Ishihara (the president of the Pokémon Company) will appear on Japanese TV on October 28th. The synopsis of the interview mentions an unprecedented game whose development started this summer. The game is said to change Pikachu, and Ishihara is worried about how the change will affect Pikachu's popularity. More details should be revealed via the interview.
Japanese fans are speculating that said change may be Mega Pikachu. This is certainly a possibility, even though Mega Evolution is generally reserved for fully evolved Pokémon. It is also possible that the game in question will be compatible with XY, and thus Pikachu's change could be carried over to these games.