It's been over a decade since The Undertaker was a villain. In fairness, the character is so strong - and has been running so long - that a heel turn has been unnecessary. It's unclear without trying exactly how many people in the audience would be willing to actually boo Undertaker, but it's an interesting premise. WWE have long said that 'Taker feeds off the energy of his 'Creatures Of The Night', so maybe that could be played into things. If Undertaker were to cut a promo saying he no longer needs that energy, it could be powerful stuff. In a sense, there will always be a large portion of the fan base who will cheer the man regardless of what he does or says, but being a heel could open up some interesting possibilities. As a bad guy, Undertaker was excellent in previous feuds and matches against the likes of Jeff Hardy and Rob Van Dam. There's room for a heel 'Taker in WWE currently. Brock Lesnar has transitioned into being a more fan-friendly character, without sacrificing his own hard-man persona. That means the top heel spot is there for the taking, and The Undertaker would be perfect. There are few better ways to reinvigorate the legendary gimmick.