If Christian does return by the Royal Rumble, there are two possibilities from this that I can see happening.
If Christian Enters the Royal Rumble match, he WILL win it.
Now before you hate this one, just think about it. If Christian comes out in the Royal Rumble, there is no other real explanation of why he would be put in this match if he were not to win it.
He would still be recovering from injury, and while he has the ability to play through the match without really further injuring himself, and even no matter how much Vince hates him, the idea of it is plain stupid.
Unless, of course, he wins it. This could set up what we all would love to see, a World Heavyweight Championship match with Edge.
He Comes Out to Either Rob or Help Out Edge
The way Edge has been going lately it seems as if he will have the title by TLC. If he does get it and retains it by the Rumble, then chances are he will defend it at the PPV.
If Christian does return to screw Edge into losing the belt, this would make a good road to Wrestlemania. (This one makes the most sense if Vince really does hate him that much, since there is no World title picture).
So then you're asking why would he help out Edge. The answer would be perfect if Edge is facing Alberto Del Rio, since Del Rio is the reason that Christian was out because he injured him and he would want revenge for it.
This too would also set up a good Wrestlemania match. The only downfall to this would be the Rey Mysterio angle. If Del Rio and Mysterio are still having their feud, then this would not happen.