I always thought the Ysalamir that Zahn introduced in his trilogy were a really cool idea. Non-sentient arboreal creatures that had evolved to repel the force because of a species of predators that used the force to hunt. Thrawn weaponized them to control a psychotic Jedi master clone he used to control his army and to fight Luke and Mara. There’s a lot of potential there for interesting force stuff to be explored.

Then, they erased all that and we got midi-chlorian counts and somehow Palpatine survived instead.