#3 "Stone Cold" Steve Austin vs Brock Lesnar
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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.