CM Punk should have defeated The Rock at Royal Rumble 2013 then gone on to defend the title in the Main Event of Wrestlemania that year.
I don't have a specific date but Braun should have won the title when he was white hot popular with the crowd.
Booker T definitely should have beat Triple H at Mania 19
The Nexus should have beaten Team Cena at Summerslam 2010 I believe it was
Cesaro should have beaten Cena for the US title as mentioned before
Sting should have beaten Triple H at Wrestlemania 31
Wade Barrett should have beaten Undertaker at Wrestlemania in 2010
Sasha Banks should have beaten Alexa Bliss on that match on Raw that she lost the title on.
Bianica Belair should have beaten Shayna Bazler for the NXT Womens title at NXT Takeover New York
RVD should have won the WWE Championship much sooner than he did at some point during the Alliance era or afterwards during the height of his popularity