Jimmy Eat World played Futures in full on the lock up in 2014 after their main stage set.
I feel like Gaslight might not neccesarily be all that likely, 2000 Trees have said they've got their 'biggest ever American headliner' which I think they would fit, and I'm pretty sure they delayed their line up release so they might have been waiting for them to announce their reunion. Also, them doing the 59 Sound in full kind of feels like a more natural thing for a headline set. Obviously they could play both but I don't see two full 59S sets at UK festivals.
Edit: Also, that GovBall hidden name is probably Lorde and another artist