I can see it happening for 2 reasons... one of which is the boring ass WWE reason of media appearances, talk shows, etc. You don't put Brock Lesnar on those, lol.
But if the plan is "Lets make as big a star out of Roman Reigns as we can",... going up against Cena is really the perfect matchup. Cena is WWE's second-biggest heel (behind the company itself) in the eyes the fans, and think of the added heat he'll get for getting his 16th title reign and tying Ric Flair's record on top of the "Wait, so you broke the fucking streak to put over John Cena!!!!?!??!!?!?!??!?!?!?!?" additional insanity - yeah, a Wrestlemania crowd would want nothing to do with Cena that night. And as much as we may root against Cena 2.0, the dude will be the biggest conquering hero since he was Not Batista at the Royal Rumble.