I'm going to say he should face Shawn Michaels at Wrestlemania. Besides the fact they're both from Texas, there's so much that can be done with this match. If they feel the need to make it a face v. heel match, they can market it as the religious Shawn Michaels, going against the Deadman from Death Valley. Undertaker can tell Shawn that he's going to bury Shawn's career at the same event where Shawn buried Flair's. No matter what they do, the matchup of The Showstopper v. The Streak would be legendary.
Another idea I saw kicked around, and agree with, is for 'Taker to face off against Jericho. Jericho seems to be involved in something already, so I doubt it would happen, but the match would be outstanding. Jericho is much smaller than Undertaker, and his offense would match up well with Taker's. It's already set as a face v. heel match, so there'd be no need to alter gimmicks, or figure how a face v. face match would work.
Ultimately, I know that they are both too big to face Undertaker at Wrestlemania, and he probably won't face someone that big until it's time for the streak to end, or he retires.
I'll put my money on him facing Kozlov. It's not an original idea, but it could work, if booked properly. Kozlov has a very power-driven offense, so it would be up to Undertaker to determine the pace of the match, and make it believable, considering we all know he's up there in age, and is on gimpy knees. Kozlov wants a challenge, and he would definitely get it here.
Of course, Undertaker has to win at the show that made him who he is today. Especially at it's silver anniversary. Whoever he faces, it's assured to be a good match, and will definitely have a big ending. I'd like to see him win with the Tombstone Piledriver. He rarely does it anymore, and it would be a guaranteed reaction from the hometown crowd if he used it, and the opponent didn't kick out.