Part 1:
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.
Not my menstrual cup!
Teleportation mimic will teleport you. To its stomach.
Breastplate mimic just wants to sink its mouth on your breasts.
Saddle mimic wants to lick your lowers parts.
No comment on outhouse mimic. We don’t talk about it.
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.
outhouse oof.
