Another idea occurred to me, this time for a rare level spawn if that’s something that’s even possible. The gist is that the level is a reflection design, with the up and down access in the corresponding mirrored halves. The level always has crystal chest rooms, one in each mirror half but a single key. Opening one of the chests makes the corresponding chest in the reflection side into a mimic chest.
If the level is a mirror level you know that they’ll be a crystal chest room on each side. You get one crystal key but 4x chests initially to choose from. Once you open one you know that there will be a mimic on the other side now, so 2x the crystal chest reward assuming you kill it in time, one from the key and one from the mimic. The point is that it’s a rare level spawn but you know you potentially get 2x the crystal chest rewards. As for the boring exploration, it’s not like the levels are big is it.
OK, so there are two iron keys and only one crystal key. I think the only way to make this work without a major code rewrite would be to utilize the new sub-level mechanic, in this case the sub-level would be the same as the floor above. But I also think if you are going to have a guaranteed mimic there should also be some kind of extra danger.