Now that the sports are over...
Thought I'd try to list all of the potential headliners for next year, including any rumours/info we've got as well as anything that could be used as a case against each act. Tried to order them somewhat from most to least likely. Apologies if I've missed any major act/got any info wrong/missed any info.
Fleetwood Mac - comments made at Wembley, someone on the farm who's ITK supposedly said they've been booked for next year. However, they've famously been out of Glastonbury's price range, what's to say that'll change for next year.
Foals - supposedly 'in the frame' to headline next year, could follow in The Killers footsteps by headlining after doing a big secret set. However, they're not really any bigger than they were in 2016 when they subbed - who's to say they won't be given that slot again rather than the headline gig.
Elton John - he's currently on his final mega tour which was clearly planned very far in advance, so could have been booked a while ago which would allude to the rumour that some 2020 headliners were booked a year or so ago. However, he's never played the festival before - maybe he's just not that bothered about playing Glastonbury.
The 1975 - probably the biggest British band to come out of this decade, make the most sense of any 'new' artist to get the gig, Emily seemed interested in Matty's comments about wanting to headline the festival, another album is on its way later this year. However, they're headlining R&L this year and its rare for an act to headline Glastonbury the year after. Not impossible, though.
Neil Young - Michael made comments about Neil Young coming back next year. However, Michael talks a lot of shite.
Coldplay - friends of the festival, obvious, easy deal to make, crowd pleasing, Emily loves them, Chris Martin was popping up all over this year, 2020 will be the 20 year anniversary of their debut. However, they last headlined in 2016 and currently haven't released any new material since then, is it too soon for them to return?
Arctic Monkeys - they're a safe bet, they've released 2 albums since they last headlined (with a third potentially on its way) and it's been 7 years which is long enough by Muse and Coldplay's standards. However, there's no guarantee that they're going to be active next summer.
Kendrick Lamar - Emily was at his gig last year and sang his praises, likely to be active with new material next summer. However, is he too expensive? He was rumoured to be R&L's most expensive booking ever back in 2018, will he take Glastonbury's low pay?
Lady Gaga - her Vegas residency will have ended by then, likely to have new material out in the next year, her stock has risen a fair bit due to ASIB, played back in 2009 so she knows the festival. However, it might be too soon after her Vegas residency to have a new show ready to tour and she might not be active over the summer months.
Blur - they're friends of the festival, would be a popular booking with the masses. However, there's no guarantee they're gonna be active next summer and there was a poster on here suggesting that they're not gonna play Glastonbury again.
Beyoncé - potentially gonna have new material out in the next year, would be a popular booking with the masses, she's a huge superstar and would be a welcome returning headliner. However, would she need/want to do it again? There's also no guarantee she'll be active next summer.
The Kinks - there's decent info to suggest that they're getting back together, they were supposed to headline the first ever festival back in 1970 but didn't, so it would be quite poetic for them to headline the 50th anniversary. However, would they headline, or would they be more likely for a sub/legends slot. Or they might just do their own gigs and not play festivals at all.
Kings of Leon - rumoured to be playing Pilton Party, which usually results in a big booking at the following festival. However, would they really be at the top of the list given all the other options? Perhaps a secret set would be more likely.
Taylor Swift - was potentially spotted at the festival this year by @CaledonianGonzo, new album is on its way and her stock has arguably dropped somewhat which could make her a more feasible headliner than she was a couple of years ago. However, she's never wanted to do festivals in the past and there's nothing as of yet to suggest that this has changed.
Ariana Grande - a very current, big star, likely to be active next year. However, would she want to headline Glastonbury at this point in her career? She certainly doesn't need to.
Florence & the Machine - could have a new album out by next summer and would be 5 years since she last headlined, which is just about long enough, she's a friend of the festival and would be an easy, safe booking. However, might be a tad too soon to return and likely won't be at the top of the list of options.
AC/DC - looking likely that they're getting back together with a new album and tour. However, they're gonna be way out of Glastonbury's price range - it's unlikely they'd take the pay cut, probably will stick to their own shows instead.
Madonna - they're clearly eager to book her, she's likely going to be active over the summer. However, she's always been too expensive and that's unlikely to change next year.
Paul McCartney - could the fuss about him playing this year actually have been miscommunication about him really playing in 2020? However, Neil seems to have it on good authority that he's not booked for next year.
U2 - do they have unfinished business with the festival that they want to try and resolve? However, they may not be active next summer and they may see Glastonbury as a 'been there, done that' gig.
R.E.M - wasn't there some whispers about a potential reunion? However, these are just whispers. Probably nothing.
Radiohead - @parsonjack spoke to someone close to the festival who reckons they're booked for next year. However, this seems far too soon to return since 2017, likely to have no new material out since then and probably not even going to be active next summer.
Kate Bush - one can dream.