Difficulty is dependent on what level you are, and would take the average level of online players in multiplayer

within a configurable distance between each other if possible
configurable +/- level range. ex: say average is level 43, with a config of -5/+10 mobs would spawn between level 38-53)

Added with 143ad2c

Does this include PlayerEx ? I don't see anything regarding it

Nope, only added levelz compat

Are there any plans for playerex ?

Not currently

How about now?

playerex support should be in a different issue, rather than using the same one. it's misleading to see an open issue for levelz support when the mod already has levelz support.