Plausible. But I could also see WWE wanting the champion to carry around both world titles on their shoulders for awhile (until after Wrestlemania where they'll likely just arbitrarily ditch the WHC) since that makes for a pretty prestigious image and that can only happen if there's a definitive winner at TLC (assuming they still wouldn't refer to the champion as the Unified Champion in the case that they unify the titles at the Rumble.) Still can't say I would be surprised to see the predictable swerve, though.
Could make for an interesting title match at the Rumble. And it's a great way to book Orton from TLC to Wrestlemania if he does walk out of the TLC match as the unified champion. He unifies the belts at TLC (and brags about his superiority over Cena and how he's the true face of the company), then he defeats the original undisputed champion (Jericho) at the Rumble, then he wins the elimination chamber match for his first time in February, and then walks into Wrestlemania gloating about how he's walking in and walking out of the biggest Wrestlemania in history as the reigning champion... but of course, he doesn't. (He either loses to Punk or Bryan, take your pick. Punk is a fresh person he hasn't defeated but Bryan defeating him brings the whole thing full circle with how it all began... if Punk wins, I expect an Orton/Punk singles match, and if Bryan wins, I'm expecting a triple threat involving Punk.)