if i was a betting man i would bet on the following legends/big names wrestling at mania31:
Definetly:
The Rock
Sting
Triple H
Brock lesnar
The potentials:
Taker
Kurt angle
Hulk hogan (tag match only)
Y2J
Low possibility to no chance:
Stone Cold steve austin
HBK
CM PUNK
Rock will wrestle he missed last year but still made the effort for the promo, i don think we wll get brock vs rock i think he will do a program with triple H or roman reigns.
If the reports on The Rock from today & from last week are true, the odds are he won't even be at WrestleMania XXXI. Fast & Furious 7 hits theaters 5 days after WrestleMania, which means The Rock will be spending heavily promoting the film and hyping it in the weeks leading up to the premiere. Earlier today, it was announced that he'll be in a buddy comedy with Chris Hart that's due to start filming right smack in the middle of WrestleMania season.
As far as Kurt Angle, he won't be appearing at WrestleMania as he's either fully expected to or already has signed a new deal with TNA. Reportedly, WWE offered Angle a full time deal but Angle wasn't physically up to it. That makes me think, and it's just a guess on my part, that they weren't all that interested in Angle returning anyhow and made a token offer that they knew he'd turn down because they know as well as anyone that his body can't take the strain of a full time schedule. So if he does sign or already has signed with TNA, he won't be at WrestleMania XXXI.
As far as Hogan goes, it's highly unlikely he'll ever wrestle in WWE in any sort of match. Hulk Hogan has said, yet again, that he's currently training for an in-ring return but even if he is, it doesn't mean squat if he can't pass the WWE's physical. WWE has very, very strict physicals for talent, especially talent over the age of 45 that was put into effect since Jerry Lawler had a heart attack smack in the middle of a tag team match. When Hogan signed with WWE, he underwent the barrage of medical exams WWE puts all talent through, so that leads me to think that they were thinking of putting him in a wrestling role at some point. However, while we obviously don't know the particulars of the exam, Hogan didn't pass and as a result hasn't been allowed to anything physical whatsoever. Hogan may well be part of the event somehow, but I see almost no chance of him wrestling.
As for Brock Lesnar, unless he suffers some sort of injury, he's contractually obligated to be at WrestleMania XXXI. If the various reports from the past few months are true, he's expected to defend the WWE World Heavyweight Championship at the event.
All of the other guesses I think are probably pretty spot on.