I'll go first. Minecart hub: Underground structure with multiple railways leading to it. Has a few golden chests with prehardmode ore and other things like pickaxes

Here's some ideas coming from me!

Edge-Caves: These would be caves that would generate on both edges of the world. Occupying the vertical space from the bottom of the ocean biome all the way to the top of the underworld. What makes these special is that for their entire height their biome type would only consist of one alt-cave biomes picked from a random selection of Mushroom, Granite, and Marble.

Boneyard: A graveyard of various gigantic bones strewn together, occupying one of the two major lava lakes in the underworld. Particularly useful as platforms for fighting the wall of flesh.

Corrupted underground lakes: These would be normal underground lakes but corrupted with the worlds evil type and less likely to generate.

What if a room spawned underground with a chest that had seeds and stuff for the opposite evil than the one your world has? it would be nice to not have to world-hop to get both evils.

Oh wow, there are quite a few ideas here! I'll add them to my doc for future reference (if I add them I'll add you to the description of the mod or smth as credit)

I got another couple of ideas
Microisland: A small abandoned palm wood hut appears on this small little island that generates as close to the edge of the world as possible while still being able to access the water chest inside (water chest generates with regular water chest loot)
Shipwreck: A wrecked ship. Has a goldEN (as in the one dropped by pirates) chest with a lot of gold things inside (no special drops from the pirate invasion though because that would be too OP.) Can spawn on the same side as the microisland

Mine of old: A place rhat resembles a mine and just have some chests with loot, no traps exept on the entrances and being near the long rais on the caverns.

maybe leaving them in a good state so you can restore them as a base, away from the center so the corruption dont reach it on hardmode.

Cabin: A small wooden building with some early game loot along with a medium cavernous hole nearby it that reaches the upper caverns layer. They can spawn on the outer forest biome, with only one spawning.

Giant Mushroom: Rare tree that can generate inside of mushroom biomes, with three per world max. Great resource for a chunk of early glowing mushrooms.

Goblin Outpost: Always generates close to the ocean and opposite of cabins. These will have a chest among a trapped watchtower with a single goblin weapon and some helpful loot.