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.
The rug under the chest is my favourite. I’m gonna give my players more trust issues, thank you
My sister and I were diving deep into the lore and rules for mimics and the most terrifying thing they could be, is either an entire planet or a humanoid. They might even be able to be a planet-sized humanoid. Or even a human-sized planetoid 😱
