As an old school fan...
Bas Rutten vs Rickson Gracie: Bas called him out numerous times, including once after a Pancrase fight with Rickson in the crowd. Instead, Rickson chose to fight Funaki, who had already retired and who was physically shot from years of pro-wrestling. I would've loved to have seen Bas obliterate Rickson and his phony invincible image along with it.
Frank Shamrock vs Kazushi Sakuraba: Frank said that he was thinking about making to jump to PRIDE and even expressed his interest in fighting Sakuraba, but it never happened. Words can't describe how amazing it would've been to see them lock horns, though for the grappling nerd in me, I actually would've preferred to see them fight in Pancrase.
Tito Ortiz vs Vitor Belfort: I know that this fight happened at UFC 51, but I wish that it would've happened when it was originally scheduled at UFC 33. I would've loved to have seen a five-round title fight, and especially if Tito would've been able to win, it would've given his title reign a huge bump in credibility.
Ken Shamrock vs Igor Vovchanchyn: This was the originally scheduled bout at PRIDE 13, but Ken got hurt, so on short-notice his Lion's Den fighter Tra Telligman stepped up and scored the upset of his career and one of the GOAT upsets. It's such an epic fight given the fact that Tra had no business winning it, but it would've been amazing as a die-hard Ken fan to have seen him get that win, which would've been his biggest post-WWF win by a mile.
Cro Cop/Nogueira II: All the respect in the world to Josh Barnett, who put on a hell of a performance against Nog in the 2006 OWGP semi-finals - the fight was amazing and Barnett nearly tapped Nog with a kneebar - but everyone on the planet knew that stylistically he had absolutely nothing for Cro Cop. Given Nog's earlier upset over Cro Cop, and the fact that Cro Cop had improved leaps and bounds in the intervening years, the only thing that would've made his OWGP victory sweeter would've been if he'd been able to avenge his loss to Nog in the finals, which he absolutely would've done and in brutal fashion considering how on fire he was that night.