#2 John Cena vs. The Undertaker (WWE WrestleMania 32)

WWE's plans for WrestleMania 32 were epic, with several major matches being planned for the supershow. It was set to take place at AT&T Stadium in front of over 100,000 fans. However, injuries to big stars like Seth Rollins, Randy Orton, and Sting ruined a lot of the proposed matches for the 'Show of Shows'.
The biggest one was John Cena vs. The Undertaker, a battle between the two biggest stars of their generation. As a match, it would have been fine. Cena was in his prime as an in-ring competitor, while Undertaker was still in decent shape. But as a spectacle, it would have been untouchable as a WrestleMania moment.
This match would have ensured that 'Mania 32 would be worthy of over 100,000 live fans, but Cena suffered a shoulder injury at the start of 2016. It would take him almost six months to recover, meaning the much-anticipated dream match with 'Taker was off. Cena did appear at WrestleMania, alongside The Rock, in a limited capacity.
Undertaker wrestled Shane McMahon inside Hell in a Cell at the event, an underwhelming alternative to his dream match with John Cena. This match did happen two years later, at WrestleMania 34, but it wasn't confirmed by WWE until moments before the bell rang and lasted about three minutes.
WrestleMania 32 would have been the perfect place for Cena and The Undertaker to finally wrestle each other, as the biggest names in modern WWE history.