The problem with Lesnar/Rock is simple...who wins? For one, they're both faces. The second big problem is that, if Brock is going to take a clean loss, why would you have it be against a guy who is, for all intents and purposes, out of the wrestling business? Brock's next clean loss is effectively a "king-maker" moment for the guy who beats him. Why would you waste that on the Rock, who at best might wrestle half a dozen more times in his life? Having Lesnar beat the Rock would be big, of course, but does Lesnar really need that win to build him up? He's already been built up as a damn near unstoppable monster who as far as I can remember hasn't had a clean loss since he returned. He broke the fucking streak, after all.
And would Rock sign on to Wrestlemania just to be sacrificed to the alter of Brock Lesnar? It would be one thing if it was an instance of a part-timer legend coming in to put over a guy on the brink (Jericho does that shit all the time, and Sting just did it for Rollins), but as I've already discussed, Brock certainly doesn't need a push when he's already at the top of the company. I just don't see the benefit for either guy, really.
What does make sense is the Rock and Rousey versus Triple H and Steph. They built up the match at this year's mania and could easily follow-through next year. Triple H and Steph can eat a loss and it won't matter because they're going to continue to be a presence on TV for the foreseeable future, and Rock and Rousey get a nice bump for coming in and getting a win. Unlike Sting, the WWE won't feel the need to make Rock pay by eating a loss in his first match since Rock was always loyal to the WWF/E.
I could also see Trips facing off against Rollins, but I feel like they might use that at a different PPV to draw ratings. I don't see them waiting until Mania to turn Rollins face. That's six months away and he's already on the precipice of a face turn. I could see it happening after he beats Kane at (presumably) Hell in a Cell.
If Austin were going to come back, he would have done it by now. He'll be 51 at next year's Mania and he had to quit wrestling due to neck injuries. Brock Lesnar is literally the worst possible match-up for him if he were to return for a match. I don't think Austin returns at all, though, at least not to wrestle. Maybe he'll be a guest referee or something, but I just can't see him stepping back into the ring as a competitor and risking paralysis. It's just a dumb gambit on his part. He quit for medical reasons and I doubt he's even remotely in wrestling shape at this point in his life.