#3 The Undertaker vs. Abyss
Believe it or not, Abyss vs. The Undertaker nearly happened in the year 2006, when WWE had reached out to the big man in TNA, although he has no regrets about his decision not to come to WWE:
I'm not going to say I wish it would have happened. If it would have happened, obviously it would have been a career highlight, no doubt, but it wasn't in the cards.
What is truly funny is that, at this point in time, IMPACT Wrestling Hall of Famer Abyss works as a producer in WWE, and one assumes that if things were ideal, the monster would have been a fantastic opponent for The Undertaker. The Undertaker has mixed it up with several big men that could not work at all, and we know that Abyss certainly could.
One wonders, though, if Vince McMahon would have allowed Parks to wrestle The Undertaker as Abyss, considering that it wasn't his creation, and would have preferred the version of Parks we saw later in his career when he embraced a more comedic gimmick.