Sting
The main thing that stopped this from ever happening was the fact that Sting was reluctant to sign with the WWE.
He was synonymous with WCW and then Impact/TNA, but once TNA was going so far down the drain, he eventually caved in and signed up with WWE on a short-deal basis.
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Since Triple H wanted to face him at WrestleMania and do the whole 'WWE vs. WCW' thing on a big stage, Sting didn't get to face the Undertaker.
It could have happened as a main-event at SummerSlam or another big PPV like Money in the Bank, but Sting was thrust into a program with Seth Rollins during his run with the Authority.
Unfortunately, during a match for the WWE Championship at Night of Champions, Rollins inadvertently injured the legend. It signaled the end of Sting's in-ring career.
But it wouldn't have been too hard to fit the match in during Sting's run since both men were icons of the same age for different companies.
Its disappointing for the fans because it would have been a huge match, instead it was another huge missed opportunity by the WWE.