If the Undertaker is going to retire in the next couple years, here is how I would love to see it go down...
I see some young star coming in and being built up, maybe from a season of NXT or ROH...someone that we are gonna WANT to see on the main stage. Have him do a slower Sheamus type build, but PLEASE no one the size of Batista, focus on someone with Orton size but with the agility of Evan Bourne.
After a few big wins, and a main stage like SummerSlam, he goes on to win the Royal Rumble ala Shawn Michaels with a low entry and lasting the entire match. With Undertaker having the title have this guy come out and cut a promo about growing up and wanting to main event WreslteMania and who better to go against than the Undertaker. Have Taker go put the title and record up. Undertaker wins and keeps his record in tact and the title. The next night the guy challenges him at the next PPV, but Taker chooses the stipulations. The match becomes a BURIED ALIVE CASKET MATCH.
All leading up to PPV, during Smackdown you see him scouting Undertaker matches and watching past videos. When the match finally comes, you have him go over Taker CLEANLY, no interference. He puts Taker in the casket and buries him under it and wins the title. The lights go out and Lightning strikes the headstone. Undertaker appears on the titontron and cuts a quick promo and then disappears. End PPV...but is it the end of the Undertaker?