What's your favorite dungeon in the series lads? Mine is wayward cave. Strange pick, I know, but consider:
>Completely optional and hidden so only people who bother to explore will experience it
>All double battles and you can do them all yourself before finding mira, or with mira after finding her, depending on which you prefer.
>Makes actual use of the samey gen 4 cave tileset in the puzzle by having repeating branching paths in a dark cave, and is actually pretty long and confusing, so you gotta remember the directions you took on the way out
>Has an entire double-secret underground section with the best fucking bike puzzle in the series (picrel) with Gible and fucking Earthquake at the end as a reward
It's simply based, I wouldn't change anything about this dungeons design or placement or execution in the slightest.