Just to be clear, I am a big Jamie T apologist (seen him 15ish times) but booking him for victorious was a misstep. Outside of London he should not be headlining main stages.
The ticketsale for Graspop starts next week Saturday. It would not suprise me in the least if Hellfest starts a countdown on Monday to reveal the full line-up on Friday (or something like that). I've got a feeling that if there are any surprise acts in store for these festivals, they'll be announced next week.