Some might not like this idea, but how about taking a cue from some of the Coil mechanics? Like the slug/slimes in T6 - have mobs that would prey on each other if you let them and they'd get stronger? Not party-wipe stronger, but enough to really pose a greater challenge if you let it happen. Or ones like the sons/daughters in T10 that need to be kept at a distance from their same kind or they get buffed? It might not be fair to party compositions, but there could be enemies that are more resistant to physical or magic damage, and I liked what Hullbreaker did with the bear traps. First time I saw them, I thought "I wonder if I can trap something in it?" Sure enough, I could.