I think a Reigns vs. Cena match will be a little further down the line. I can see it as something they save for SummerSlam or possibly Survivor Series with the selling point being Cena is back, once again intends to be the top dog and to finally become a World Champion for the 16th time.
I think the only real chance this match has is if Reigns is a heel by the time it goes down. Yes, yes, I know WWE could go the route of turning Cena heel but I think most of us have long since given up on that. I can see it being sort of a "pass the torch" sort of deal, but I don't think this match comes off right without someone being a definitive heel as neither of these guys are fully embraced as babyfaces by WWE fans.
As I said, it could be meant as some sort of passing the torch sort of deal, I'm sure some of us will interpret it in that light whether WWE markets it that way or not, but such a thing probably has to happen at some point. John Cena's 39 years old, he's long since done everything there is to do in WWE, he's made a huge pile of money, and he'll be WWE World Heavyweight Champion at least one more time before he hangs up his jorts.