While it would be unprecedented to announce Generation VII before Volcanion is distributed, I don't think it would be that big an issue -- especially if the latter is revealed earlier (through Movie 19 details or potentially the supposed spinoff).
The fact is that this is the most sensible option if a second 3DS generation is to happen. Granted, it's not necessary this is the plan, but if it isn't, it does raise the question of whether Game Freak is inclined to release an NX game within a year of its debut.
In any case, only time will show. I wouldn't preclude a sequel significantly more content-heavy than B2W2 (regardless of whether it does anything to freshen the metagame), but I do think a third version would be too little, too late.