Punk vs. Undertaker for the belt would be pointless, since 'Taker only wrestles once or twice a year now, so it's not like he is gonna suddenly go back to working full time to defend the belt (no way will Punk be the one to end his streak; he doesn't need the rub). And having him be the one to end Punk's long streak, only to drop it a day or a week later, would also be pointless.
Sadly, I think it is a given that Rock beats him for the belt at the Rumble and then faces Cena again at Wrestlemania. Which means this is all but set up to put the belt on Cena again.
I'd love to see them keep the belt on Punk for a few more months at least. It reminds me of several of Ric Flair's title reigns, where he always somehow manages to not lose the belt. In essence, it is like Flairs's 6th NWA title reign, which saw him as a face for the first nine months, and then he became a heel and held on to the belt by crooked means for another five.