Can't see past Arcade Fire after they didn't do a secret set after all. It'll have been a good few years since they headlined by then and they're the most consistently brilliant band (both on record and live) out there at the moment (this is subjective of course but also objectively and extremely true).
tl;dr AF FTW
I guess two years gives us a chance to see if the explosion in grime's popularity was a fad, if Stormzy is still going strong with the yoots by then I see no reason why he shouldn't get given the nod. I wouldn't mind seeing someone like Foals promoted, perhaps pending another decent album release between now and then. Something in the mould of What Went Down, which is full of powerful riffs to make ya go 'mmmmmm ye' in a big crowd.