Or it could be surprise Gen 7, I mean who the hell expected Black and White and Black 2 and White 2 so close to the end of the DS's life span. It's most likely Z due to those Zygarde formes though. Though even then with Regigigas appearing a generation after the Regis it wouldn't be too out of left field to be gen 7, but again it's probably unlikely.
There's also been a rumor of a trademark for a code name Pokemon Rainbow, code name mind you most likely not the final name, but I doubt it because said leaker said he got the info from a NoA employee in the marketing department who also gave him the marketing budget information for the 3ds and Wii U as well. I would think an employee wouldn't risk that considering what happened with that Smash Bros leaker way back, though dumber things have happened.