It would be fairly straightforward to bring Edge back as a face. All they need to do is cast their minds back to 2001 when HHH got injured. At the time, he was one of the top heels, and then he comes back to an incredible reception and got a pop for beating the hell out of Angle, who everybody loved to hate at the time. Then he goes on to win the Rumble and beat a heel Jericho for the undisputed title. Granted there was the stuff with Evolution after that, but for a good while he was a top face.
I think I remember the promo videos they had as well, I'm sure they went for one of those 'we have the technology, we can rebuild' kind of things. That could work in this instance as well.
So lets say for arguments sake that he's out until just before WM26. You can forget about him main eventing in a title match in that instance. They may even wish to hold him back until after WM, which could make more sense (or they could just lazily throw him in MITB). Let's also assume that the WWE holds a draft just after WM.
They could work some kind of angle whereby the returning Edge is being fought over by the three brands in the weeks prior to his return (obviously ECW would just have a token role to play). So say the night after WM on Raw, Orton's out in the ring with the title after retaining the night before against HHH and Cena part 27. He's talking about how he's sick and tired of proving that he's the best there is on Raw, and that surely, there has to be someone other than HHH or Cena to challenge him. Edge's music hit's, he announces that he's coming back to Raw, and well, the angle just runs itself from there. You've got the old Rated RKO teammates battling it out for the title, with Edge getting the pops from beating the top heel on Raw. The only trouble then being that we'd get Edge v Cena. Again.
Or they could work some kind of angle involving Jericho. Say they work it as if Jericho thinks Edge is taking the spotlight from him in their partnership, so he beats the living hell out of him and 'puts him out' for a long time. Fast forward to WM and Jericho is in the Smackdown main event in a no DQ match for the WH title. Jericho has whoever he's facing (say Hardy, assuming he's still around), locked in the walls of Jericho, with it looking like he's going to tap. All of a sudden, Edge's music hits, and he comes out to get vengeance on his former partner. And this is where it gets interesting, they can even have him beat down Jericho and cost him the title, or swerve and have him reunite with him.
So, there you go, just a couple of ideas that I think could work.