There are Mimics for wood, shadow, and gold chests, but none for dungeon chests. My idea is for Mimics to spawn rarely, like paladins in the dungeon in hardmode, before and after plantera, disguised as (locked or unlocked) dungeon chests. They can be disguised as corruption, crimson, hallow, snow etc. chests. They have a higher chance to drop key molds (of the chests they're disguised as) than other mobs. Normal mob's chance to drop a key mold of the biome they're in: 1 in 2500. Biome Mimic chance to drop a key mold of the chest they're disguised as: 1 in 100. They would be minibosses, and have 3000 health, 20 defense, 60 damage, and drop 5-10 gold coins. They would use the Mimic AI.