That is to say, Arenas, Treasure Vault/Trove rotations, and perhaps a few mini boss levels, like the boss from heart of ice, could be placed so that they fall on depth 28, as well as before each terminal/subtown, with plenty of boxes as a reward.

This, along with designing levels based on where they will fall, only feels natural and would promote healthy runs. You wouldn't place Vanaduke on depth 26, would you? Of course not.