#3 Cain Velasquez vs Brock Lesnar 2
While we did see a 'rematch' of sorts between these two long-time rivals over in Saudi Arabia, it was done under the constraints of professional wrestling. If they want to do this rematch in a way that could also bring in some casual MMA viewers, then sanctioning a legitimate scrap (as difficult as that may be) could be the direction they go in.
It isn't exactly outside the realms of possibility, with some even suggesting this could've happened over in the UFC before both men walked away from the promotion. The ending to the Saudi Arabia showdown between Cain Velasquez and Brock Lesnar wasn't exactly viewed as a satisfying way to conclude their rivalry, and you just know WWE wants to cash in on it.
Pretty much everyone has wanted to see Brock test himself inside the cage at least one more time, especially after there were so many rumors a few years back that he was going to make the jump back over to the UFC in order to fight Daniel Cormier.