solar cobalt
I propose something similar to the look of the Star Fury room, which I loved, and call it something like the Fungal Pit. Thematically it would make a lot of sense as a lower level after the Arboretum. Same as the other two levels, curse at the start, relatively short, a darker palette, but with beams of light coming in from the ceiling throughout. Dripping water, those destructable platforms from the Shipwreck level, and some hazy sections could work well too. I would make it relatively a straight line so the light coming in from the ceiling makes sense, with short side paths going up and down here and there. I would have the lower side paths sometimes be mid depth pools because the underwater section of the Castlevania DLC was great and I want more of that. You could introduce a second type of mushroom mob or create a stronger palette swap of the Yeeters and Jerkshrooms, perhaps in yellow for an easy Last of Us easter egg in a lore room.
If you want to create something totally unique, you could have a "chase" style section from say the halfway point after the shop, where a mold fungus style growth would start popping up behind you and force you to race to the end a little like the fire in the lighthouse but sideways instead of up.
That said, I think there's definitely room for something in that space, and the prison depths doesn't really fit imo.
quartz bone
empty vigil
slim saffron
tender cradle
foggy inlet
alpine prawn