Well the TV writers are trying to catch up to GRRM, so I have no immediate fear that they will run out of ideas this season.
I will say that Martin has been working on the next book for a long time, it is apparently huge, there is no announced date of publication, and the next TV season will probably require that the book be out.
I think there is a chance that Martin actually has writer’s block and simply cannot complete the next book. That has happened before, to other writers.
He’s released or read several sample chapters at conventions. The problem seems to be that he’ll write a chapter, but then he’ll keep re-writing it.