Undertaker's suspiciously heelish actions at SummerSlam didn't go unnoticed. The Deadman benefitted from a screwy referee error, low-blowed Brock Lesnar to gain an advantage, and gained an incredibly dubious win over the Beast as a result. With their rubber match at Hell in a Cell supposedly the last ever bout between the pair, don't be surprised to see Taker go fully heel in an attempt to scrape a victory. I don't personally think he should win via such tactics (Lesnar can't afford to lose again), but could at least go on a destructive heel rampage after the match. Have Undertaker try every vicious trick in the book to win, lose anyway, and swear vengeance on the entire WWE. A few months of sporadic terror could ensue, until a babyface hero steps in to stop the madness and challenges the Deadman to a titanic WrestleMania 32 showdown. That's Adam Blampied-style booking right there.
