From what I've seen they're going to do something a bit different from the Cornetto movies. Pegg said on a podcast a while back that they have an idea that they'd like to work on but have had to wait a few years for them both to become available to write it. For the other 3 they've rented a flat/cottage for a couple of months to sit down and write.
Always LOVED the third personally, though it's certainly not on the level of SotD and HF. Not that it has to be.
Pegg and Frost have already done that!
The Cornetto films were based on the idea of putting ordinary/"boring" British locations/characters into a Hollywood style story, which I think is what they're wanting to get away from in the next one.
Whatever it is, I cannot wait.