This definitely seems like a match that Cena is going to win, but it's really a win/win scenario. When it comes down to it, the WWE didn't have anything good for Cena here. After his injury, they knew they had to re-establish him quickly lest the top draw start to lose his luster. So they had to give him the world title. With plans clearly in place for unification, okay, he obviously feuds with Orton then. Again, since they were going to push the Authority hard, it meant that Orton had to win. In another time and another era, one where perhaps Daniel Bryan didn't become a phenom, it would be Cena in this storyline with HHH and Orton, looking for one last chance after getting screwed time and time again. But Daniel Bryan could not be ignored, so he's Mania's top face. The sum of a series of logical moves and with too much momentum to change them is that Cena was shut out of the title picture.
Now, you then figure that next to wrestling for the title, the next biggest thing you can do at Mania is wrestle Undertaker. So why not Cena/Taker? Certainly people wanted it. But then, the WWE is locked into this contract with Lesnar that only gives him so many dates a year. Obviously, one of those dates has to be Mania. And if not Taker, then who in the world would Lesnar wrestle? He's already wrestled Cena, and what sense exactly would Lesnar/Cena II make? Not to mention it leaves Taker out to face, uh...nobody special. Maybe Taker/Wyatt could have been done, but that makes even less sense at face value than this.
So, with both of the two biggest things he could be doing taken out of the picture, Cena was the odd man out to pair off with a rising midcarder and do the dance with. With Wyatt solidly on the rise, probably scaring the shit out of the 8-14 demo that forms Cena's strongest fanbase, it seems like letting Cena kill the boogeyman is the next best thing. While Cena will undoubtedly win this match, I think like many of the people Cena wrestles, Bray will come out looking like a much bigger threat. I suspect we'll see the Wyatts focus on the tag titles after Mania, personally, but this should be a nice chance for Bray Wyatt to shine and have everybody look at him as a bigger deal going forward, and it's a solid chance for Cena to have a big match to at least some part of the WWE Universe.
I expect we'll see a hard reset of continuity post Mania (probably with DBD as champion) and Cena will make a return to the main event, but for now, this little diversion into Cena as an upper midcarder is pretty decidedly interesting.