i think there are 3 ways to go about this. he could have one more feud with his brother kane and go out that way. wwe could put him against an up and comer and elevate them to the main event status. (eg. bryan sheamus punk ziggler etc) or they could leave him at 20/0.
it all depends on the undertakers fitness and if the ratings buy into it.
Wade Barrett or Sheamus, I'd really like to see somebody who shows potential to be a top star for the next decade, facing Taker and beating him to get that mega rub. I know a lot of people want to see Taker cling onto his spot but lets face it the bus is pretty much done as far as being an active wrestler so he might as well give up the only thing he has left to help the future of the industry. The thing with Taker as well is that he's totally business and seems like the kind of guy who will want to put somebody over at Mania by laying down for them, knowing that he's made a megastar as he steps away from the ring.
Orton - He hasn't had a feud with UT as a big main eventer after their matches in 2005. 2012 Orton is much different: mult-time WWE Champion, main eventer, etc. He's already challenging the streak and came really close.
I have no interest in Lesnar, since that's an obvious outcome, no newcomers like Barrett or Sheamu. Sorry, they haven't earned it yet. And I've never had any interest in a WM match against Cena. I know I'm in the minority here, my pick is, and always has been a match against Orton, but I know it won't happen.
I don't want to see Taker wrestle anyone at WM. I think 28 was so perfect and a fitting end to his career but I doubt his career is over. If I had to guess I'd say Taker/Cena.
What Id like to see is Taker V Wade Barrett. Have Barrett win the WHC from Sheamus at like Summerslam and carry the title until Rumble or the EC which would be about a 5-6 month reign and establish him as a top heel. During that time just have Barrett destroy everyone like Orton, Sheamus, Cena, etc. REALLY take the time and build him up as a badass heel. Have Taker cost Barrett the title at Royal Rumble or EC and challenge Barrett to a match at WM the next night on RAW. I dont think this will happen but hey who knows.