A couple of things.
First of all, unless Taker is seriously in serious bad health, he will wrestle at Mania. He's obviously getting plenty of time to rest, heal and treat injuries. Reports were last year he was in bad shape and still delivered ten fold in his match with Trips. So if he isn't really really fucked up, he will be wrestling at Mania. This is WrestleMania. The Undertaker is one of the biggest selling points of the show. So again, unless he is in really bad shape and pretty much physically unable to compete due to his health, he will be at the show. Thanks to all the time off he will be in better shape and even if his health can't warrant a return he can still have his one match at Mania.
Forgot who mentioned Taker being thrown in with Rock and Cena but that's just not going to happen. Taker vs. Triple H 3 isn't happening either, if it's anything involving Trips it will end up being Taker vs. HHH vs. HBK. But it's the slimmest chance of that happening. His best bet for his opponent would be Punk(If Punk/Austin doesn't happen) or Chris Jericho. Jericho talked awhile ago about returning, pretty decent chance of him returning a few months before Mania. Taker/Jericho won't touch the scale of Rock/Cena or Austin/Punk but it can very well be the match that steals the show. It's still a big match though and definitely would be one of the biggest matches on the card and obviously a main event.
Now there is other potential matches but they simply will be a huge step backwards(not able to compare to the matches with HBK and Triple) however they can still be a solid match on the card in terms of everything. Del Rio would be the best bet but Barrett, Miz and Christian all have a chance of being the opponent. I highly doubt a match with any of those guys will happen but if it's just going to be your average WM match at Mania, nothing huge like the past few matches Taker has had, there is a chance with either of those guys.
The last thing I want to touch on is the streak. Undertaker's WM streak is simply one of the biggest accomplishments in professional wrestling. It's something Taker has earned for all of his work. It's a huge deal, ending the streak would result in the biggest win in wrestling and the biggest push. Even if you're John fucking Cena, you will become a much bigger name. Obviously if the streak ends, Taker can no longer hold an undefeated streak at WM as his biggest accomplishment but his reign of being undefeated will always be a big deal and he simply will still be remembered as one of the greatest to ever step in the ring. The three issues from ending the streak are killing his biggest accomplishment, the amount of heat/hate that will be caused by it and whoever ends it. The first issue shouldn't be much considering again he will still be remembered as one of the greatest, the 2nd would die off eventually or perhaps be controlled if it is actually done right and the "right" guy ends it. But that's the biggest issue. I could name off a few guys I'd like to see do it but not everyone would be in favor of it and whoever it is can really mess it up with even the slightest fuck up.