I honestly think this is where we're headed: Barrett wins the WWE Championship at Survivor Series; Barrett subsequently loses the championship to either The Miz directly or we get Orton winning it back only to be cashed-in on later (TLC, Royal Rumble), doesn't really matter because in the end The Miz is WWE Champion sometime between the end of Survivor Series and the end of Elimination Chamber.
For Wrestlemania, we get Cena vs. Barrett, with the entire Nexus saga behind it, in a non-title main event match (the actual, genuine main event of the evening). Orton vs. Miz is just underneath it for the WWE Championship. I have no idea about the World Heavyweight Championship over on Smackdown but I hold out hope that it's Kofi Kingston challenging... perhaps Alberto Del Rio... for the Big Gold Belt...?
By that point, Cena and Barrett won't need the championship to be even part of, much less the centerpiece, of their storyline. That'll be the payoff, blowoff, whatever you wish to call it, of the entire Nexus storyline, which will close with Cena being triumphant in victory at the very end.
Cena vs. Undertaker will be saved for Wrestlemania XXVIII, when Undertaker is trying to get to 20-0.
The only fly in the ointment is that Nexus buried Undertaker. So they could easily spin that off into Taker vs. Barrett or even Taker vs. Cena at Wrestlemania. But that doesn't mean it's nailed down for certain. Taker could return and cost Barrett the championship indirectly by beating him up, laying him out and inadvertently giving The Miz the opportunity to cash in on Barrett, leading to a Royal Rumble or Elimination Chamber match-up between Undertaker and Barrett. Taker could even put Barrett over in a high-profile grudge match in the winter in the lead-up to Wrestlemania, to give extra momentum to Barrett going into his Wrestlemania match with climactic Cena, especially since Taker would be a lock to go over at Wrestlemania against whoever they want to pair him up against (Sheamus? Who knows?)
In other words, I have no idea, but I'm assiduously trying to figure it out ahead of time!