Sarcasm aside, I genuinely didn't see it coming this soon. I thought they were going to do the same thing they did with Keldeo and hold on to it for one more year. Then again, Victini was present in the Reshiram/Zekrom movie, and I tend to forget about it because of the Liberty Island launch event (as well as its unusual Pokédex position), so I guess I misjudged everything.
With that said, now that we know Diancie is going to be in the next movie, we can assume Hoopa will be in the one after that, and then Volcanion. So supposing this generation lasts four years, which is a reasonable guess, what's going to be in the last gen 6 movie? A gen 7 Pokémon, perhaps (like Lucario and Zoroark did before)? Or are three-year cycles the new standard for Pokémon? That's a question worth pondering.
Edit: Mega Lati@s, of course. Why'd I forget about it? So if that's the case, gen 6 will probably be four years long again.