I mean Rihanna is (google tells me) 1.5 Billion. The diference between a Glasto payday and a BST one is a good as pocket change for her at this point I doubt money would be a factor in her booking its probably if anything what does she want to do in terms of legacy and I should think Glastonbury thumps BST when it comes to that.
I think when you factor in the fact that they had her in the bag to a certain extent in 2023 before having to pull out to be replaced by GnR. This is her first opportunity to play since then. A new album and a world tour is confirmed.
I would say it's hard to not see her as one of the most obvious candidates.
She has achieved about all you can in music and a Glastonbury headline slot is about the only big thing you can say she has not done.