There have been a lot of rumors floating around over the past few months that current WWE World Heavyweight Champion Brock Lesnar may become a free agent after WrestleMania 31. WWE would like to re-sign him, but Lesnar does apparently have some interest in returning to mixed martial arts fighting and both the UFC and Bellator promotions are rumored to be interested in signing him.
As of right now, Lesnar's future after WrestleMania 31 is not certain, but the UFC promotion might be the most likely candidate to get Lesnar because WWE is in a situation where they may not be able to afford to offer him the same type of deal he's had since he returned in 2012.
But there is also some speculation that Lesnar might just be using UFC and Bellator to hopefully get a good offer from WWE. So at this point, there is definitely still a chance that Lesnar may just end up staying with WWE.