The "Dream" opponent would have to be Sting... It's probably the one that everyone would want to see most, simply because it hasn't been done. But, I don't think it's likely.
I have 3 "possible" opponents... These are three guys who could be possible, for a 1 time match and would make for a very good match, IMO. Two have been mentioned.
1: Mick Foley. The history is there and I think he's the one who has the most victories over Taker in his WWE career... Kane might have more victories, but it's close and Foley has never gotten a shot at Taker at WrestleMania.
2: Steve Austin. He's been a lot more involved lately... Maybe it's just to hype Tough Enough, but something about tonight's RAW showed me that he could go one more time, especially facing Taker with Cena/Rock on the same card. Those two matches alone would sell WrestleMania XXVIII.
3:Bret Hart. I don't know his current medical condition, but if he could pull it together for a "street fight" match with Taker, that would be an interesting fit. Remember, Bret was the first WWE wrestler to lock up with Undertaker in a WWE ring.
Ultimately, I'm thinking HHH gets the nod. If they go this route, to really up the ante, I'd like to see two conditions:
1: Streak vs Career. If HHH loses, his career will officially have "died trying" to end the streak.
2: Shawn Michaels as the Special Referee. He either has to count Taker's shoulders down for a 3 count at WrestleMania (something he could never do) or he has to count his best friend's shoulders, effectively ending his career.
After the past 3 Taker WrestleMania matches (2 vs HBK and last night vs HHH) being so well done, I think they need that extra boost of Michaels as the referee in that position to put the match over the top. At this point, IMO, they wouldn't be able to put on a good enough match to be on par with the previous three, without something that REALLY thickens the plot... HBK as the referee does that.
Just my thoughts.
-Bill