John Cena isn't hanging up his boots anytime soon. And when he does, guys like Wade Barrett, Alex Riley and Curtis Axel will no longer be seen as "the superstars of the future" anyway. Cena should retire at Wrestlemania, but it certainly won't be in an Intercontinental Title match.
I do like the idea of Cena winning/competing for the belt though, and putting someone over in the process. There are ways to work it without centering it around a retirement angle, though. A couple ideas:
1.) Maybe Cena is told by Brad Maddox or whoever is the bastard GM by this time that if he loses the WWE Title (or fails to regain it) in an upcoming match, he will no longer be able to compete for the WWE Title (or the World Heavyweight Championship) ever again. Remember the stipulation of the Austin/Undertaker First Blood match at Fully Loaded 1999 that said if Austin lost, he could no longer wrestle for the WWF Championship again? Same thing. Cena loses, and then he can wrestle and feud for the Intercontinental Title for a couple of months or so before finding a loophole back into the main event (ironic thing is that Austin was supposed to do this with IC Champion Jeff Jarrett back in the day as well, but he didn't want to work with him.)
2.) While Cena is WWE Champion, maybe he has a side feud going on with the IC Champ. Remember when Christian was knocking Cena back in 2005 after Cena won the WWE Title for the first time? Christian was on Raw, Cena was on Smackdown, but they were still teasing an eventual feud between them by having Christian take open shots at Cena. They could have someone do a similar role now, a mid carder who happens to be the Intercontinental Champion who starts knocking Cena until eventually Cena confronts him and puts him down (in a segment, not a match.) Maybe he starts knocking Cena by just pointing out to people that he's a better IC Champ than Cena is a WWE Champ, I dunno. Come the title match at the PPV, the IC Champ can cost Cena his world title against someone for revenge, and as a result, Cena drops down into the mid card for however long to feud over the IC Title. He says he's never been IC Champion and since this guy felt the need to cost him the world title, he'll now return the favor by taking HIS title, the Intercontinental Title, and putting it around his waist.