I don't see it.
1. I think both guys are done. With the streak over, Undertaker should retire. Have a HoF moment, but that's all that should be expected of him at this point. Likewise, I don't think Austin is healthy enough or really driven enough at this point in his life to have one last match.
2. We've seen this match so many times. SummerSlam 1998. Rock Bottom 1998. Over The Edge 1999. Fully Loaded 1999. Judgement Day 2001. Backlash 2002. I'm sure there were others as well. But without a gimmick like First Blood or Buried Alive, these two great workers never had great chemistry together. The matches were average back then, I don't want to see what they'd be like now.
The WWE (If in typical Vince fashion want to push name power over new stars) they have a few potential big time main events to play with over the next couple of WrestleMania's. Any one of the following, or a combination of the following, could main event WM31 or 32:
Sting vs John Cena
This is more likely than Undertaker at this point, and I could see this happening for WM31, to be honest. Especially if they're not sold on Reign's drawing power.
Sting vs Triple H
Likewise, if Cena is involved in something else, subbing in Triple H would also be a marquee matchup for Sting's likely one and only WWE match. If Sting is only wrestling once, it makes sense for it to be against an established name.
Brock Lesnar vs Roman Reigns
This is the rumoured WM31 main event. They're building Roman up to be the next big star, let's see if he gets the biggest spot on the biggest show.
Brock Lesnar vs The Rock
This is the other rumoured WM31 main event, which if you believe the reports, Vince feels would be a much bigger draw than Roman Reigns. This could be saved for WM32 perhaps.
Roman Reigns vs John Cena
It may be too early in Reign's push for this match to feel big. But it could be perfect in a year or two as the two "faces of the company" go toe to toe.
Roman Reigns vs Seth Rollins vs Dean Ambrose
Probably more of a smark pipedream at this point, but this match is bound to happen at some point. Maybe more of a SummerSlam or B PPV main event though than WrestleMania calibre, at least for now.
Personally, I'd like to see a combination of the above to sell the next two WrestleMania's:
WrestleMania 31:
Brock Lesnar 'C' vs Roman Reigns - WWE World Heavyweight Championship
John Cena vs Sting
WrestleMania 32:
Brock Lesnar vs The Rock
Roman Reigns 'C' vs John Cena - WWE World Heavyweight Championship
Also, considering how big the stage will be at AT&T Stadium, I don't think they'll be selling 100,000 tickets. Closer to 80,000 probably.