Here's what I think
After the Unova League, and a few fillers, this will be the end of Ash's story. THE END.
Then what will Pokemon have till they announce something else? They could make Black and White specials focusing on Hilbert/Nate and their adventures. I'm not asking for a high quality animation like the trailer, but an anime just like they did with pokemon all this time. With a solid plot, they'll be able to stretch the episodes including flashbacks and a few fillers until GF announces another game.
Something similar to Pokemon Crystal won't hurt.