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I've often thought it'd be fun to let a party have a tame mimic as a pet. Not helpful in battle unfortunately, unless one of your opponents decides to take a looting break.
I think in the early editions they said that mimics were only really effective at imitating wood and stone. I agree that mimic chests are done to death but "anything could be a mimic" would get old fast I fear. Limiting them to only some materials but then getting creative within those limits would keep it interesting without it feeling like a cheap jumpscare.
But, if we're going to get into DM advice, the way I'd recommend stopping "anything could be a mimic" from getting old is to have it constrained to a themed side-adventure, or a one-shot. For example: A wizard tower where the guy's one weird hobby was breeding and training mimics. In such an adventure, you want to start fairly tame, but towards the end, the more outlandish and ridiculous the better.
As for the constraints on mimic forms, most of my DnD based jokes use 5e as their basis, as that's what the majority of my audience are likely using.
Oh totally. I didn't mean that as a criticism of the comic, I really like the comic. Your artstyle is very distinct and charming. It just got me thinking about how to run mimics generally. A mimic wizard one-shot sounds like a lot of fun.
Cover your phb in spray adhesive and leave it sitting on the table. As soon as someone touches it, shout "ROLL INITIATIVE!"
Actually, apply this to other random objects at the game table. A bag of chips, 1 can of soda in the fridge, every 3rd pencil, whatever.
Mimic city; every building is a mimic, but they're smart enough to eat the local population when they're alone or in small groups. Every time people keep going missing, and the players will assume it must be some random wandering monster; they'd never think it's literally the buildings and each room can start eating really fast. There'd be no pattern either, every building is alive and people go missing literally everywhere. Perhaps a clue can be a house that has no more people living in it and it starts to starve to death and starts freaking out, even endangering the other mimics' secret.
I never got to run this, but I remember after playing Prey putting together a dungeon themed off of mimics and getting really nasty with them. I imagined that after a couple rooms the players would adopt a policy of preemtively fireballing every new room before entering it.
Abomination Vaults has a mimic encounter where one is pretending to be a door and the other is pretending to be a weapon rack with a nice axe on it. Gets them either way.
You might also want to look into Rotgrind's Mimouthouse. It's exactly what it sounds like.
having the table you are irl playing at turning into a mimic and having to roll stats for your irl person in order to continue sounds really fun
