It really is sad to think that just about everyone our generation grew up on won't be around for too much longer. Rock and Austin are gone. HBK will be hone next year. I'm sure it'll be the same story with Taker. I'de honestly be shocked if we see him last until next Mania. HHH is about to...or already has turned 40 so who knows how much longer he'll be around.
As for who HBK's last opponent should be at Mania that really is a tough one. I would have to agree with Mike and saw perhaps one of the top heels in Edge or Orton....more so Orton though.
It's sad for us, but new young fans don't give a shit. The same way that when most people on this board came up watching wrestling, we were "stuck" with Rock, Austin, HBK and Triple H, and people before us were feeling sorry for us that we didnt' get to see the prime of Dusty Rhodes, the Von Erichs, Ricky Steamboat, etc...
It'll be a sad day when his career ends, but it'll be sad for us who watched them, not for the new wrestling fans who haven't.