Sex Pistols would headline if they reunited, no doubt about that. Slightly pointless to keep going on about booking them though when they haven't been active for 10 years. You might as well say Led Zeppelin could headline.
In terms of 'fresh' realistic headliners they could book in the near future: NIN, Tool (done it before I know but they haven't toured the UK for ten years) QOTSA, Pearl Jam, Green Day, BMTH, Blink 182, Fall Out Boy, 30STM, Paramore- some would undoubtably go down better than others
The likes of Architects, Ghost, Enter Shikari, FFDP, Parkway Drive that should be on their way up but need to avoid over saturation for the next couple of years.
Then there's the old guard; Korn, Manson, Deftones etc. that aren't quite big enough but will probably end up getting the slot once Metallica, Maiden etc. have all retired.