The Nexus
When it comes to feelings of 'what might have been,' you need look no further than The Nexus.
The Nexus were originally contestants on NXT, which wasn't a brand so much as a reality show competition at the time. Wade Barrett won the competition, but unexpectedly joined with his former rivals to form the Nexus and run roughsod on the entire WWE roster, including John Cena.
Heel stables aren't normally comprised of all rookies, but the Nexus worked. They clicked together and worked well as a unit.
In the eyes of many, WWE bungled the Nexus by having them all fall prey to John Cena during his 'super Cena' booking era. They tried to reform several times, under CM Punk and as the Corre, but the magic was gone.
Still, for a time it was one of the most compelling angles in wrestling, and made enough impact to be included on this list.