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Possible Showcase matches. These matches were hinted at but not 100% confirmed yet:
Kane (with Chyna) vs Triple H
WWF WrestleMania XV - March 28, 1999
Kurt Angle vs Shawn Michaels
WWE WrestleMania 21 - April 3, 2005
Undertaker vs Brock Lesnar (with Paul Heyman)
WWE WrestleMania XXX - April 6, 2014
21 matches in total (and possibly a bonus match or several).