It's hard to compare anything WWE could do today to a match up that happened when the company was at the height of its popularity. Nothing today can really hit Attitude Era numbers.
That said, a Cena/Bryan feud with Cena as a heel could be HUGE right now, especially since Bryan owns a victory over Cena for the title on one of WWE's biggest pay per views. A victory that Cena has even brought up in his promos more than once, so he clearly hasn't forgotten.
If WWE really thinks Reigns is their guy (even though we all know it's really Bryan), they might as well pull the trigger on Cena. If they were to have Cena go heel on Bryan on the next Raw, not a single goddamn person would complain about Bryan not being in the title match at Mania. Bryan versus Cena II would be a MUCH bigger talking point.
However, a Cena heel turn seems like one of those fantasy scenarios that will probably never happen, much like the idea of WWE ever giving Bryan the ball as their top guy.