If possible, you could add an option to increase the spawn chance of an eldritch mob depending on the distance to the spawn point. With this, distant regions will be more likely to spawn difficult mobs (making these locations more challenging to players).

This migth be achieved by adding a setting to define the block distance as increments starting at the world spawn point. For example, if setting a 200 block distance, an elite mob have a 0.01 chance to spawn if its from the world spawn point up to 200 blocks. From 201 to 400 blocks, the elite mobs spawn chance increases to 0.02, from 401 to 600 blocks, the chance to spawn is 0.03, and so on.

I havent checked if it's already incorporated, but an addon or alternative to this suggestion could be increasing the spawn chance depending on the dimension (such as doubling the chance of an eldritch mob in the nether). I feel this could be a bit more complicated to integrate with other dimension mods though.