I could see Edge and Christian teaming again, especially if Edge returns as a face and Christian gets drafted to Smackdown. But I think it would be better not as a steady thing, but more of an occasional or short-term novelty (maybe even give them one short title reign), kinda like the DX, Hardys, and Brothers of Destruction of the past few years. But the two being completely dedicated to the tag division together for a prolonged period of time would be a considerable step backwards for both of them.
Sure, Edge became tag champ with Jericho before becoming injured, but they probably just wanted to take Edge away from the world title picture for a little while, much like they did when he teamed up with Orton. However, the difference is that when Edge teamed with Jericho and Orton, neither had been done before. Aside from that, Rated RKO only lasted a few months, which is probably about how long Edge and Jericho would have lasted. If Edge and Christian were to team up for more than 2-3 months, that would be too long for them to stay at the point they were at when they both first started in the WWE over a decade ago.
That being said, in the event that Christian were drafted to Smackdown, given the choice between them feuding or teaming soon after, I would choose teaming. Christian wouldn't have been built enough to get into a full-fledged feud with Edge at that point, especially with the way their feuds have gone in the past and the level Edge is already at. The only way it could remotely work would be if they really dramatically played up the idea of Christian having come a long way from the little punk he used to be, with him cutting strong promos against Edge expressing that sentiment.
As far as how it would end if they were to team up, I'm not sure why so many people seem to think that Christian should turn heel on Edge. I say Christian should stay a face to build up his credibility until he can rise to his full potential. Besides, he hasn't even been back that long, people still wanna cheer him. Aside from that, just because Edge and Christian would be a repeating tag team doesn't mean they need to repeat the exact same storyline from 8 years ago. If anything, have Edge turn heel on Christian, but better yet, I would be just as glad to see neither. Not every tag team has to have a dramatic, resentful breakup. Why not just find another way to have them go their seperate ways?
In closing, am I excited about the idea of Edge and Christian reuniting for either a feud or a team? Definitely...if done right.