I don't care for this either. Even comparing their gimmicks isn't really applicable because Sting hasn't been the dark kind of mysterious Sting since like 1997/early 1998 (unless he pulled it off at some point in TNA that I missed since they set up shop in 2002) and Undertaker is getting closer and closer to becoming his humanized "Dead Man" persona from the mid/late 90's (especially 1997-98) more and more these days. This never was a big dream match for me anyway. To be honest, I would have loved to have seen Sting in WWE, but there's not really any particular guy that Sting could work a feud with that would make me think "omg, I can't wait." I've always just been a moderate Sting fan at best except for when he was stalking and owning the NWO throughout late '96/'97.
Unless they really want Taker to go to 25-0 (a risk, in my opinion), I don't think a Sting match would even be considered for Taker at Wrestlemania at this point.
I also wouldn't call Undertaker working a few matches even post-Mania this year and wanting to work another one at Summerslam as proof that he's wrestling past WM next year (in response to Gohan's second post.) If anything, it could simply be that Taker has already decided to call it a career at WMXXX and so figures he can take the workload of working a few extra matches to entertain the fans and in the case of The Shield, put a few young guys over in process.