Seen some very strange ones in this thread so far, but then again, maybe the same could be said for my stab at it below: MOST LIKELY:
Arcade Fire / Mumford & Sons / Arctic Monkeys / Kings of Leon / The Killers (I'm sure at least some of these will be back in the future) New headliners (Long time big acts who've never done it)
Madonna (Have Glastonbury ever tried to get her? Does she even play festivals? Unless there's some reason to believe otherwise (?) - I can see it happening eventually - I think she's canny enough to realise the good will and extra sales she'd get from it)
The Stone Roses
Elton John (I know he did Bestival a few years back, which is obviously smaller, but is there any real reason why he wouldn't headline Glastonbury? Has he never been asked? He's got so many hits and he can still sing. That's good enough for me...)
Depeche Mode (Ok, they're much bigger internationally than in the UK so would probably be quite suited to the Other Stage, but if Glastonbury got them to agree to headline the Pyramid - and have I read right that they have tried to - why not? Would strike me as similar to the Neil Young booking a few years ago) New headliners ('breakthrough' acts)
The xx - If a new album is as well received as their previous two, The xx will absolutely be ready to step up and headline. Just look at the amount of Spotify plays they've got if you doubt that (and we're not just talking 1 or 2 songs), and then add their critical acclaim, and the fact they've already proved themselves with headline slots on smaller stages/smaller festivals.
Kendrick Lamar - Kendrick Lamar is certainly on the right trajectory - though it could be a while. He hasn't exactly got the casually-known hits of Jay-Z or Kanye West yet, but if he isn't already, its clear that he's going to be the most important guy in hip-hop for years to come.
Ed Sheeran - I hope its not next year, but ye, fair play to him, Ed Sheeran will likely end up headlining in the not too distant future.
Taylor Swift - Same goes for Taylor Swift too I guess.
Big acts who've not headlined before - less likely
Red Hot Chili Peppers
Rhianna or Lady Gaga
New headliners ('breakthrough' acts)
Tame Impala - Tame Impala are bigger than I realised, and from the sounds of it, their sub slot went down much better than most would have guessed. Maybe subbing the Pyramid is their peak, maybe it isn't - but I don't think we can say never.
Biffy Clyro - Biffy Clyro have definitely gone backwards in the UK and very likely missed their chance with Glastonbury now, but another massive album and it is possible, its just whether that will happen though.
The National - Seems more likely that The National won't ever break through to that level, but if the next album was either very critically acclaimed or a surprisingly big commercial success, they could be given the chance to step up too. They'd have earned the exposure such a headline slot would bring.
Queens of the Stone Age - Queens of the Stone Age would be a big curveball, but again, if a new album was very well received why not? Would be more exciting than just booking bands who've headlined several times before just because they're available. They'd give a cracking performance and would deserve it really.
Dizzee Rascal - If Dizzee Rascal releases something to trouble Album of the Year lists, rather than just the charts, again, maybe he'd deserve a punt. He's British for a start, and Glastonbury have booked him often enough to know he can pull a crowd and put on a good show. Looking a bit further ahead:
Chvrches - Very early days so maybe they're a silly prediction - but if ever a band could be bumped up to that level and really nail it, I think Chvrches could do just that. I really did think they were fantastic on the Other Stage this year. I hope they're more than just a very good flash in the pan.