He'll either be a supprise returner at MITB, but can't see there being one at Summerslam now unless its one that could be cashed in on either belt. Otherwise you'd have presumably either Ziggler or Rhodes with one WHC MITB and Barrett with another.
They've been getting Ziggler so close to the world title for a while now and teasing face turns, the crowd is often behind him so I could see them blowing the lid and turning him face.
Have him announce he is cashing in at Survivor Series on whoever the champion is. Long match where Ziggler wins the title, only then for Barrett to cash in and become champion, Ziggler and Barrett then fued over the winter. After that fued Sheamus (just guessing) is awarded his rematch for the WHC and after Barrett gets a screwy finish (pin via pulling tights or something) match is made Sheamus Vs Barrett for Mania for the world title. expect WWE would bill it as the battle of britain or something.