1) Jericho and Big Show were broken up, but they've continued keeping them together, and now they're getting a rematch. How much do you want to bet that they win the titles back? This leaves DX without the titles and able to split.
2) DX/Hornswoggle is horrible and they can't possibly keep this up.
3) With Bret Hart here, you can't really make it so he's the heel. Nobody will boo him. Granted, they could easily go the route of McMahon being the heel, but if HBK isn't involved it's screwing the fans out of what they really want to see. If given the chance, do you REALLY think anybody will prefer to see Hart vs McMahon instead of Michaels? I don't expect Bret to wrestle at all, but even just a verbal argument is still in favor of HBK.
4) If they do go ahead with HBK against the Undertaker at WrestleMania (which I don't think they will) then the only way to have HBK lose for a second time would be for the fans to WANT him to lose. I can't really see them making Taker drop the streak.
5) HBK doesn't have that much time left in his career. As much as I hate to admit it, since he's my favorite of all time and the one I truly consider to be the best professional wrestler in history period, he won't be around forever. Thus, he may want to have one last run as a heel, and when you deal with someone like HBK, how do you turn him heel and get the fans to boo him? Well, there are only two people that you can possibly turn Michaels into a villain: Undertaker and Bret Hart. Every single other person on the roster would just be overlooked as HBK is above them in the eyes of the fans.
It could be great if it happens. Heel HBK is always fun to watch. I just hope we don't get HBK vs HHH at WrestleMania, as I think it would work much better if they saved that for HBK's final match period (similar to the Flair angle). However, if it comes down to it, I'd much rather have HBK vs HHH with Sheamus not on the WrestleMania card at all, instead of the rumored HHH vs Sheamus for the WWE title. Anyway, we have about 6 days before the potential for a heel turn can be realized...so we'll have to see.