He didn't beat Bray in the run-up, Bray beat him at the Rumble (clean I might add). Maybe he beat Cena at SummerSlam '13, but Cenas elbow injury can still be included if they want to book a rematch. Bryan hasn't beaten Reigns and Brock.
He has beaten Orton, HHH and Batista. Two of them are part timers. I don't think Bryans choices have been exhausted at all. And cherrypicking the two guys (Punk & Bryan) who got something out of dozens of Cena opponents who got buried, is not a good argument for Cena.
He beat Bray decisively in a cage match on RAW a few weeks later (at the end of the Daniel Wyatt idea).
If he wins the championship - he will (most likely) have to beat Reigns (at Fast Lane, or as part of the triple threat at WM) and Lesnar (at WM).
The problem is that there are not many full-time main event heels
Faces: Bryan, Reigns, Cena, Ambrose, Orton (when he returns)
Tweeners: Lesnar, Reigns
Heels: Rollins, Lesnar?, Sheamus (currently injured), Wyatt.
If Sheamus is still injured, and Lesnar leaves after WM; you have two Main Event heels. Wyatt and Rollins. That's it. (Big Show and Kane do not count, for the love of all that is holy).
I would not be surprised if Rusev actually goes over Cena - they might use Cena to create a new top level heel (they still have the "Cena has never lost to submission" card to play if they need it). If you're going to go with Bryan, you basically would need to turn Reigns heel and have Rusev go over Cena to give Bryan even some semblance of credible heel challengers.