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You say in the description of your Elemental Mobs pack that you have other custom mobs and bosses. You should consider selling those, too!
You should add in a few more well-known horror movie villains like Jigsaw, Michael Myers, Freddy Krueger, Jason Voorhees, and Chucky!
Perhaps you could add a Magma Spirit variant for rocks and the earth, and another variant for the sea.