This sounds like something that could be both heavily abused and produce awesomeness. I think it would work best as part of a balancing system. Meaning something similar to Spore where you can only add so much to the creature. Because just as there are surely specially modded 'super hardcore' worlds that are night all the time and can spawn mobs at almost any light level and such, someone would make a flying, water-walking, charging ,exploding, light-tolerant, fireproof, spider-skeleton-zombie hybrid hyper mobs. Tho on the other side someone would probably make very interesting variant mobs to help vanilla become just that much more Minecraft. I do think that the pack mobs would be very interesting but scary idea.
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