Oh there are plenty of things to write about. Maybe the Age of Apostasy. That would do for an interesting horus-heresy style series of books.
The Scouring would be a natural 'next' after the horus heresy series, if the Black Library were to continue in such a manner.
I agree that there's plenty of things they could pull stories from, but would they even begin to come close to what is presented in the HH? There's plenty of smaller stories that can be told forever, for sure, I won't argue that one bit.