|Chad Brown Pathfinder Adventure Card Game Developer|
Imagine if the card required you to roll 1d4 and based on the roll you would get:1) Nothing, the chest was empty.
2) 1d4 of random weapons from the box.
3) 1d4 random items from the box.
4) Summon and encounter a random monster. If defeated, you may draw a random armor from the box.
I'm not saying that this idea is impossible, but take a look at the few cards we've made that have tables on them (Ogrekin, for example), and think about what this kind of card would need to say.
As a practical matter, we'd be more likely to give you a chest that did #4 on that list than have it be one of four possible outcomes. Something like "Summon and encounter a random monster from the box. If you defeat the summoned monster, summon and encounter 1d4 random items from the box." comes closer to fitting, but even that concept is missing some important mechanical support.
In summary, there are a lot of good, interesting ideas that are hard to pull off in a card game. As a guideline, if you can't fully express the idea in two tweets, then it probably won't fit on a card.
That said, I have a few different ways that we might see a Mimic. It's on my list. :-)