If they're going to continue the trajectory of having a massive popstar each year, then Taylor Swift, The Weeknd and even Drake I can see doing it. I think for Dua and Billie it depends on how well their next albums do - I can see Billie smashing it out the park with her new album and getting a headline slot from that, I think for Dua it depends on the strength on whatever comes after Future Nostalgia, because I wouldn't have pinned her as a headliner from her debut alone.