I'm going to have to go with an Edge VS. Christian feud.
Yes, I'm very well aware that these two have had a feud before, but it was over a mid-card title, and it was really a one-off feud, when you consider that the only meaningful match these two had was the ladder match at No Mercy. These two are capable of having a long, six month feud over any of the world title's of their choice. These two have history together. Hell, they were one of the greatest tag teams, at one point. And remember that segment that these two had at Backlash? The fans still realize the implications of a feud between these men, and it got one of the bigger pops of the night. They're destined to have great matches... I don't think either man would consider anything less. Christian seems to be playing the role of a face extremely well. And yes, Edge is leaning towards tweener status at the moment... But that just seems oh so boring. I love seeing Edge as a heel. And perhaps this could be his last feud before coming back as a heel.
Here's how I imagine it... Remember when Shawn came back in 2002, and Triple H teased reforming DX? It's a very similar thing. Edge comes up to a newly drafted Christian (whatever brand they're on. Doesn't make much of a difference to me), and welcomes him back. He hands him a pair of those huge sunglasses, and Edge asks him to be his tag team partner that night against... Well, anyone, really. Doesn't matter all too much to me. Anyway, they tag, win their match, and give everyone the great reunion that some people have been waiting for. They hug, Edge raises Christian's hand, things along that matter.
Next week, they're back to regular E&C... Edge channels his inner 2000, and becomes to bleache blond incarnation of Theodore Logan. He tells Christian that they're going to the ring tonight again for a shot at the tag team titles, but before the match, he wants to do a five second pose. They come out in some type og gimmicky stuff to represent the town. They do their typical Edge and Christian banter on the mic, and give the crowd a good laugh. When it's time for the five second pose, Edge strikes at Christian. He spears him, and procedes to grab two chairs. He gives Christian a con-chair-to, and takes Christian out of action for, I don't know, a month. At least until the next pay per view.
Next PPV, Edge has a shot at whatever World Title. Doesn't really matter to me. Christian costs him the belt, and teases to do a con-chair-to Edge, before some heel mid-carder comes out to attack Christian. We then have Edge and this mid-card heel going against Christian. Personally, I'm biased, and I've already said how much I love Dolph Ziggler, so I'd use him. Dolph could act as an enforced to Edge, making sure Christian doesn't get to him until the time is right. Then, when this feud has hit its crescendo, you have Christian beat Dolph, earning a match against Edge.