I remember seeing a Glastonbury interview with BMTH last time they played and they responded to a question with something along the lines of 'Yeah, we're well happy to have worked so hard over the last few years, to come back here and play the exact same slot...' so I can't see them coming back unless they're offered something big - I agree that the festival probably won't offer them what they want!
I agree that Skindred & Sleeptoken would go down a storm. Maybe not metal, but I can see Don Broco going down well too! A few years since Enter Shikari have appeared I think, maybe they will show up again with new music?