By 2017 the 7th gen should definitely be out. If they keep the 6th gen as short as the 5th, we get gen 7 in 2016. Since XY mention a new region already, I see them releasing a new game with a new region unexpectadly soon... who knows if they won't call it gen 6 part 2 anyway? o-o
I mean, I could totally see "Z" being "gen 6 part 2" with like 50 new Pokémon (no new starters), some of which are legendaries. Kind of a "delayed release". Also new mega evolutions of course (maybe even for all starters lacking one... at least the Hoenn ones).