The Weeknd is about the only one of those I'd concede on, and even then he's been booked before on West Holts. Although that was a good while back and he's obviously a much bigger deal now. At this point there's been nothing from either side indicating that talks have happened and stalled. Doesn't mean they haven't - we just don't know.
Taylor, Gaga and Hazza will probably all do it at some point, with the Hazza being a very likely proposition next time he tours. I don't agree that Taylor wouldn't come back for purely financial reasons.
Bieber and Bruno the festival wouldn't be that bothered about going after. They'd be very weak choices.