#3 "Stone Cold" Steve Austin vs Brock Lesnar
This match is probably the one on the list that came closest to actually taking place. During Brock Lesnar's first run with WWE, he naturally had a lot of momentum behind him. That momentum was supposed to take him to a marquee match against Austin on Raw in 2002.
The Texan believed a match of that magnitude would be wasted outside of a PPV, especially with next to no build. The disagreement led to the Rattlesnake leaving the company for a while and hence the match never took place.
When Stone Cold returned, he didn't stay long and retired after a match with The Rock at WrestleMania X9. However, The two have clashed and Austin even refereed The Beast's match with Goldberg at WrestleMania XX. They never wrestled a match, though, and they obviously never will.