[Paladin]

To date I think the biggest missed opportunity for any Toku series was "Dino Thunder."

BOA came up with this great new "action figure pack" to sell the other Dino-Zords but left out the Cephala Zord for apparently no reason, meaning any American collector for this series would have to import unless they wanted a one-armed Ultrazord.

As for non-toku stuff...

Transformers ANIMATED! This series had so many excellent designs that never saw release, with my #1 missed opportunity of this series going to Omega Supreme.