Yeah, the timing seems way off to be generation 5. Considering that DP just came out, getting people excited about the future now seems like a bad marketing strategy that would draw focus away from their hot sellers. Further, by the time they release the new games, the excitement over the new Pokemon will have faded - two generations would have been weakened for the price of one revelation!
It's more likely that the next-gen Pokemon will be revealed in the 11th or 12th movie, methinks.