So a mimic. The epic feature best known for mimicing chests and tricking adventurers. Cool, right?
We could just y'know, make an NBT tag on the chest block entity (instead of adding a new block which could just be detected with WAILA), then check that NBT tag on chest open.

But a mimic... it's more than that. It's a MIMIC. It mimics things. Not just chests. It could be that wooden door in the stronghold. That unusually placed bit of gold ore. It could even be your sword, or an axe.

Mimics are adhesive. They grapple their enemies, preventing them from moving with their sticky coating. This substance is also a solvent, with the mimic's digestive enzymes. This could be included in the combat.

Mimics are somewhat more intelligent than some other creatures, and neutrally aligned, and thus that sticky mimic axe may even choose to be a companion, a weapon you can carry around. But who knows, it may backstab you one day if it feels it can benefit. You never know with a mimic.

But seriously who doesn't want to carry a shapeshifting axe around that refuses to be dropped from your hand due to its stickiness

As this mod wishes to be able to work server side, this should be an optional feature, enableable in the config

Interesting suggestion. This will be shelved until 1.17 is released, as I won't be adding new blocks or items until then.