Basically, one can use a new item called essence of mimicry which has no recipe to set a chance for a mimic spawn egg to spawn inside a chest or other kind of typical storage container.

If it does happen the chest converts into a mimic when opened but the mimic contains the loot of the chest it over took.

alternatively if the loot table contains a mimic spawn egg the same happens and the converted chest mimic will drop an artifact also along with the loot it would've denied when it converted the chest.

If I understand this the right way, basically what you want is a way to configure structures to sometimes contain mimics instead of chests, which would then drop the chest's items instead? I could probably add some way to do this, although I'd have to put some thought into this first. (This likely won't be added in the near future either way, as I'm working on some other stuff right now)

Doing it through loot tables like you suggest using a new item doesn't quite seem like the right way to do this. Among other issues, players might be able to obtain the item if they manage to access the container's loot in some unexpected way.
I'm also not entirely sure how I feel about making the containers transform only when you try to open them, I'd rather keep the behavior of mimics consistent with how they work underground.

mostly that is the intention of the idea, yeah. To have chests that can come alive on you to keep people on their toes. Since you could just inject a separate table with just the essence on a percentage chance and thats what would trigger it.

But for the shonkey it's probably as you say, they'll yoink the essence and such. But even if someone were to get the essence all they'd be would be an annoyance or one time troll/trap thing they couple pop on someone since the items would still pop out of the mimic once killed.

it was mostly just a passing idea, since I would figure having mimics in other structures capable of generating would add some extra level of spook to just running in and looting other than me using locks mod to have them be locked so you need to secure the area first.

I think the idea of having mimics in any structure has potential but I don't understand what this essence item is supposed to do.

thats just the way to facilitate it that I came up with. rather than doing it for every structure automatically and without choice.

With the essence item being a marker of sorts it checks for when opening a chest which can be placed via loot tables edited for a modpack it lets you choose what structures of what tables you want to be convertible, and also being a table you can specific the chances differently for all the different chests you do tables for and their structures.

The item existing in the chest when it's accessed just makes the essence delete itself and then converts the whole chest into a mimic on the spot. the mimic when it dies then just drops any other items that the chest contained.

then, if people really want, you can use the essence in creative or add recipes to allow people to trap chests as gotchas in maps, presets, pranks etc by just slipping the essence item into a chest then not opening it again until an unsuspecting person comes and triggers it.

alternatively there can be stronger mimics also if you use different grades of essence to make them appear. so unlike the default mimic that always drops an artifact, these will use the mimic AI and stuff, but they're made to only drop the loot their spawn chest had contained, but cause they're mobs they could be modified by other means, like champions and such for... evil pack devs and such.

kinda like how chests spawning in certain fights in final fantasy 10 I think it was, when stolen from would sometimes turn into mimic creatures that'd kick your but.