I don't see it happening at Wrestlemania 29, or at any event in the foreseeable future. TNA reportedly signed Sting to a newer contract recently, and that alone locks him up, probably through WM 29.
Also, I just don't think Sting is dying to come to WWE. He's had countless opportunities, countless offers, and at the end of the day, each time WWE has tried to sign him, he's found another offer, or reason to decline. Yes, he is one of, if not the last of the 'icons' that is still working steadily in the business, and he hasn't faced Undertaker, but bringing in someone like Sting for a match with Taker, especially a WM match where the Streak should be center stage, is a bad idea. Sting would overshadow Undertaker, The Streak, and the mystique and allure it brings to Wrestlemania each year.
Also, WWE is on to something with the idea of The Streak matches tying in to Undertaker's storied WWE career. We saw it with Triple H, Shawn Michaels, it creates a kind of tension, and allows for a degree and depth of storytelling that a non WWE talent, with no history with Taker can bring to the match at this stage in Taker's career.
Lastly, while I respect Sting, I'm not sure he can 'go' at the level someone should be able to go for a match with Undertaker at Wrestlemania. Yeah, they could dress it up with stipulations and some added distractions, but Sting has really gone through the paces over the last few years, be it no fault of his own. He's had injuries, he's not the young man he once was, and I'm not sure he's in a place where a 30 minute match with Undertaker, which these days are spot filled, high tempo matches is something he's fully prepared or ready for.
I'd love to see Sting in the HOF, and I'd love to see him find a way in the door with WWE, but I don't think it's with Undertaker, and sadly, I just don't think it's going to happen.