...What feud do you look forward to happening?
I think a feud with Mark Henry could be great. It would be new and fresh. They could milk a couple of months out of that one. Have the first PPV match end in DQ or a No Contest and then have Henry destroy Cena after the match and cause some internal bleeding and/or taped ribs leading to the rematch, where Cena manages to overcome him. An alternate idea is having Henry take the belt in the first match and injure Cena for a few months, ala Earthquake taking out Hogan in 1991 (which had kids crying back then, too) and then having Cena come back and win it in the big comeback match later.
A face/face encounter with Ryback that ends with Ryback taking the belt could be neat if they built Ryback up to have some sort of aura but I'm afraid they've made him a choke artist by having him lose every big match he's had to date. I'm not even sure if a victory over The Shield in some capacity will make much difference.
Another idea is to toss a midcard/upper-midcard heel in there with him for a month. Sometimes, it's cool to test the waters by putting someone in a title match even though they're clearly not ready for a world title run, but it's a way of seeing how the audience react to them in that situation. See Austin (against Undertaker) at Cold Day In Hell 1997 or Cena himself (against Brock) at Backlash 2003 or Jericho winning the belt from Triple H in April of 2000 as examples. Or Ryback last year at HIAC, though they were kinda forced into that one. Throw someone like Damien Sandow in there with Cena for the belt for a month just to see how people react to it. Could even do a false finish where it looks like Sandow just won the belt (just so fans can see what it looks like to see Sandow holding the belt) only to restart the match and have Cena retain. There's reports of Vince wanting to see Sandow in the main event scene sooner than later anyway.
Those are just a few ideas.