The National played the last show of their tour last week, so they're unlikely.
Given that Edge is a Royal Blood fan I wouldn't be surprised if they popped up. Perhaps The Joy Formidable, HAIM, Band of Skulls...there's a whole list of really good young bands out there right now, U2 should be looking at it.
Royal Blood is opening up for the Foo Fighters (a much better fit, IMO).
The Joy Formidable would be awesome: Ritzy is a fireball on stage and I think their songs would really come across well in a larger setting.
I'm actually seeing Royal Blood open for Foo Fighters at Wembley in June, looking forward to that. I've caught bits of The Joy Formidable's sets twice (at Reading Festival 2012 and supporting Bloc Party at Earl's Court in 2013) but I've never caught a whole set of theirs. Need to do that.