I disagree... If you listen to any interview that he does he's always talking about staying in shape (not ring shape, but at least staying healthy and working out). He's not THAT old, and since he's been off for a few years he's had more than enough time to heal up the nagging injuries that he had at the end of his run. As much as I would love to see HBK wrestle again, it would go against everything he's said. When he lost his retirement match to Undertaker he said he would never wrestle again. He didn't want to be like Ric Flair, retire and keep coming back to wrestle again.
What I think would be cool, is to set up a situation where Bryan wins the Royal Rumble (as has been hinted), but have Triple H as champion heading into WrestleMania with the ultimate storyline conclusion being Bryan beats H for the title at WrestleMania. Have HBK in Bryan's corner (playing on Jose Lethario in HBK's corner at WM12). I know nobody really wants to see Triple H as champion again but I think this would make for a great conclusion to the whole "Best for Business" for Bryan to prove that he is not a B+ player...