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.
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.