I love the idea of Challenger Mobs but having them spawn from Spawners makes it way too easy to get plenty of the rare loot they drop by setting up a simple spawner farm.

I suggest a way to make it so Challenger Mobs only spawn naturally and not from Spawner Blocks. Preferably configurable?

I think right now this won't be configurable because of the means by which the event is triggered. I hope that isn't to much of a deal breaker!