There are a few different theories that point to Pokemon's next main series game, Pokemon Sun & Moon, being the final one in the series, but I think it's really just the final one before they literally start over again.

This could be supported by the fact that the game is seemingly throwing everything it can at the table to make this game completely different from all the others, including getting rid of gyms and doing constant callbacks to generation one by giving many of the original 151 Pokemon new forms. In fact, they just announced that the Day and Night system is making a return.

There's also the fact that the number of Pokemon is nearing 1,000 and just how impossible that might be to manage if the series just kept stacking. A 3D remake of the original Red and Blue/Green series just seems inevitable, especially since they already gave us a version of this with the Omega and Alpha series and the Heart Gold and Soul Silver games before it.

I very much so expect Sun and Moon to spell the end of this version of Pokemon.