So something I was thinking of that would work with the chance value on when an NPC can spawn, a value that checks for the instances a specific NPC can spawn.

Here is what I mean. If I had a L4D Tank as an NPC, I would lower the chance of spawning to about 30%. but even with that, there is a possibility for a bunch of them to spawn, over runing you. So, to combat this, there would be an option that would only allow a specific amount to spawn at one time. So from 20+ tanks spawning, I could lower it so that only say 3 can exist at once.

What happens after you kill one is up to debate: Do they spawn in again instantly when it is their turn to spawn, or is there a cool down for how long it takes for one or all of the tank NPCs to respawn again?

The values that would be for the instances would be 0-whatevernumberyouhaveformaxNPCs, where 0 would be the default setting and everything would work like the addon does, or any number like 5 which dictates that only 5 instances of an NPC can exist at a time.

TL;DR: The ability to control how many specific types of NPCs can exist at a time, and possibly the ability to control the time it takes for them to spawn in again.