Unless Sting really is gone from TNA for sure, then the odds aren't great. These rumors seem to pop up at around the same time every year. If he IS gone, then there's a decent chance it could happen. Sting has talked before about how never participating in at least one Wrestlemania is the only thing missing from his career, plus he's named Undertaker as his dream opponent.
I doubt Taker/Cena happens this year because it just doesn't seem headed in that direction given Cena's involvement with Orton and The Authority. Plus, they want Hogan to have some sort of affiliation with Cena this year (either as his tag team partner or the guy in his corner) and that seems odd for a match with Taker. I'm getting a typical "good trumphing over evil" scenario, with Hogan there to be apart of it. I suspect that they may wait until WM32 because they're booked at the Cowboys Stadium that year, which can pack over 100,000 seats. On top of whatever other matches they'll end up booking, Taker/Cena being saved for that year could help them break their WM3 record of 93,000+ people.
On top of a match with Lesnar, Taker/Bryan has been rumored for this year as well, for the record.