If Cena's still holding the WWE Championship, then I hope he drops it to someone at Summerslam. That would be the perfect place to put over a new young guy.
If Cena's not holding the WWE Championship (unlikely, but I'm just thinking of the scenarios), then I can see a big main event involving The Shield vs Cena and 2 other guys (again) in some gimmick match.
CM Punk's apparently returning around Summerslam. You have to think that he'll be involved in something big around or even at the event if that happens.
They're talking about doing Taker vs Lesnar at Summerslam at the moment. It's a main event match, but likely won't be THE main event, since that has to involve Cena.