The only artists that have really been rumored this year:
Britney
Taylor (most likely declined due to Coke deal)
Bruno Mars (probs true because it would be unfortunate)
Fan speculation:
Coldplay
Carrie Underwood
One Direction
There's a Britney Spears insider who has been leaking factual information for well over 7 years now that already confirmed she had been offered the Super Bowl but had not signed on it yet. With Britney having the final say, the chances are incredibly low that she'd actually do it...though she is releasing her lead single in February