Here are some ideas and suggestions put here since the experiment on discord has ended.

Items:
-A swiftness charm based on how empty your inventory is(empty slots eg).
-An Aurora shield that can only protect from magic(breaks if you take physical and needs to recharge).
-A luck charm that gives a small chance to add some loot to never opened chests.
-Elemental Variants on the Peacock Feather Fan(such as boreal or fiery).
-Aria Charm, gives water breathing as long as you don't do any actions except moving.

Locations:
-A spider infested tree, with cobwebs and spawners, cocoons and shenanigans. Could also be in the dark forest depths with spider queens.

Creatures:
-Drowned souls that appear underwater in the lake biomes. Would act like twilight wraiths, they would be trying to make you sink.
-A siren-like creature in a lake cave. Could even be a boss and a small dungeon.

My favorite idea here is the twilight lake improvements. I think that they're currently pretty bland and some sort of mini boss would hugely improve the biome as a whole

While a tree full of spiders and more things for the lake are things we might and want to do, the rest of the suggestions are a not quite things we're currently looking to add in their suggested forms. Because of that I'll be closing this issue, but we might see something from here in the future.

All of these are ideas that I supported during the experiment! Something to make underwater exploration/underwater ruins more accessible would be great -- perhaps an area-of-effect underwater breathing? I'm not sure how water breathing works these days.

Reworked the aria charm, should be similar to what you meant 👌