I will agree, I think if Rihanna for whatever reason were to perform for the halftime show, she'd really have to step up her game because she really isn't all that much of a performer. Not criticizing her, cuz i do enjoy her singing and she's a great singer, but i've noticed in her performances she just stays in place the whole time or when she does move, it's just walking.
As for Coldplay, I could actually see them performing. I don't know why, because i feel like they don't have enough well known songs, which can become complicated with the amount of time given to perform. But i also think that they could possibly perform considering the past couple years it's been pop star after pop star.
For Katy, I feel like she would have a fantastic halftime show! As most of us have seen through the CDT, PWT, and every other performance, she really puts on a show like no one else. And she has more than enough well known songs! She would also definitely bring in quite the numbers for the NFL because she is so wildly popular. Hoping she gets it!