Me likey. What would prevent people from bypassing parts of the dungeon by tunneling through it, though? Puzzles mean nothing if you can simply bypass them easily (the jungle temples are a prime example).
Four cubic chunks in size isn't very large, though. That's basically a 32x32x16 rectangular prism.
Though I do think that the prefixed items should be a little different from regular items besides having the possibility to break the level cap on some enchantments. Perhaps allow some normally-incompatible enchantments to occasionally exist on the same item, like potentially getting Sharpness and Smite on the same sword.
Also, some more prefixes:
Strange (definitely not a TF2 reference)
Unusual (definitely not a TF2 reference either)
Netheric (i'd say hellish, but there is no hell in Minecraft)
That's all i've got for now, some of them are probably pretty mediocre but i'm just throwing ideas out there.
I guess it could have a mob that gives you the mining fatigue effect similar to the Elder Guardian, but that might be a whole other suggestion.
For the suggestion as a whole, It sounds good. My only complaint about it (and any other floating generated structures) Is that there would be weird shadows and lighting glitches on the ground under it. You could fix that with torches, but it still looks weird.
Really great idea, just thinking that maybe you should have to trigger the temples flying ability. Or maybe you'd have to pull a lever to get through into the dungeon itself, which would also cause lava to cover the cube. Anyways, I like it so - SUPPORT