For example, rather than targetting the player specifically, you could turn the mod into defense game of sorts, where the mobs will search and destroy certain blocks which will generate resources during the night or even during the day under certain conditions, which when active they will call hordes to spawn and gradually path towards that area, sort of how invasions occur in villages and the illagers come to the center of the known village area and hunt down the villagers in houses etc.

Ability to have hordes of other mobs not just zombies and undead? For example maybe creepers, wildens from ars nouveau and other such things.
This could tie into the first suggestion where you can have different kinds of defendable objective blocks that generate different kinds of resources and will call different varieties of hordes and invasions to fight you.
Adding to this you could make a horde be "captained" by a boss, of which there will only be one. Either say an apotheosis boss spawn, a set of configured mobs (mutant zombie? mutant skelly? mutant enderman? an evoker? etc)

In the far future if you're feeling adventurous:
Take some thoughts from the great and powerful corosus? Particularly the mob stacking/pillaring or mining, which can be accelerated if mobs spawn with equipment.
https://www.youtube.com/watch?v=pIs_eiWcs5A
https://www.youtube.com/watch?v=v09E1q7IIgs