Twenty One Pilots headlined every festival they played across Europe -- really don't like them, but they couldn't be called as co-headliners. It's just a coincidence they played Sziget before Foos, maybe it was the only possible date. If TOP and Foos headlined different days, both of them would sell well.
It's complicated thing about UK visitors -- last year there was Dua Lipa, who never headlined any festival before and yet after Sziget, and her day still was sold out. On the other hand there's The 1975 who also played by daylight or on the second stage at every festival (apart from UK ones). Sziget decided to promote them as headliners to engage Brits, and feels like it was the emptiest day of this year's festival. So I don't know if they take a risk and make one day's lineup mostly for UK people tastes like this year. In case they will Stormzy probably take the headline spot (with strong support act from UK).
Can't see there old legends like U2, Bruce Springsteen, Fleetwood Mac, Paul McCartney, The Who, Bob Dylan, Paul Simon -- they'll take a big chunk of budget and I don't think Sziget target audience are into them. Metallica and Rammstein probably will continue their stadium tour (Rammstein already is). GnR are playing almost 4 hour long shows -- when should they open main stage on their day, 2PM? 🙃
You're probably right about other categories, but they still could mix rap and pop day (with The Weeknd), or book two electronic headliners (one EDM DJ and one like The Chemical Brothers). Let's wait for first acts of 2020 festival circle...