It's important to remember that Cena is that he's 36, and he's built up some serious injuries over the last decade, most notably the broken neck. 36 is the same age Edge's injuries started to catch up with him. Cena has been going pretty much non stop for 11 years now, and it's difficult to tell just how long he has left. Obviously, if it were up to Cena, he'd wrestle forever, but I have to feel there will come a point where the doctors tell him to slow down, start taking time off.
With that said, it's difficult to tell when exactly Cena will no longer be full time, so I think, from now on, that he should start helping to elevate younger talent when he's not fighting or holding the WWE championship. A good feud with Cena, if booked properly, could do wonders for guys like Bray Wyatt and Dean Ambrose, who have plenty of heat and main event potential, plus have never been seen as jobbers by the fans.
So yeah, that's what I'm hoping for. Rather than just throwing him into a feud with Del Rio or Big Show or Kane, do something a little different and have him work with a younger, future main eventer. Although, eventually, Cena will do his stuff with Undertaker or Punk or whoever he's facing at Wrestlemania.