I just saw a zombie villager stuck in a hole, turning left and right over and over again. It reminded me of the 'Robot' dance.
So I had an sudden idea: What if there was a generally neutral, but sometimes hostile mob that was essentially a mechanism powered by Redstone?
You could find them deactivated in ruins and be able to power them up using a Redstone charge. Or maybe even using a Redstone Battery you can also find in the ruins?
They could be found roaming different biomes; generally they are neutral, but can be "rewired" using switches to have them guard or patrol an area marked by "Red Posts".
They may also be turned reactivated and turned hostile to the player if they walk on a pressure plate trap.
Just a simple concept, but I think I could potentially just be scratching the surface where it comes to implementing them.