This is a list of suggestions that are not worth having their own issue. It's just a bunch of ideas, feel free to add, give feedback or whatever. Just putting them here so I don't forget them tbh.
If we decide to work out some of this, I'll probably describe it better.
P.S. Layout is a bit f'd up due to github being a weird stubborn bitch.

Mobs:

Ambient

• Mudskipper
  Small mobs that spawn on silt or mud beaches. They are slow and have little health. They could drop poison glands (so they can be obtained in peaceful mode)
• Swamp Plant
  Basically the regular swamp plant, but with a weird creature under it. When a player nears, they jump out of the ground, yell, and run to bury themselves again somewhere else. They drop the swamp plant block.
• Chirpsect
  Medium-sized insects that can be found on the higher branches of giant weedwood trees. They are actually not mobs, but blocks (like the fireflies and cicadas in the Twilight Forest.
  When a player gets close, they open their shield and glow, while chirping a peaceful sound. They can be obtained with a net, and placed by the player.
• Jellyfish
  Can come in different sizes. They are quite static, they float majesticly in the swampwaters, mainly in the Deepwaters biome. They are colourful, and glow a bit. They do sting when you get too close, a bit like a cactus.
  When killed they have a chance to drop Glowing Jelly, which can be used to make glowsticks, a throwable-waterproof torch.
• Big Jellyfish
  A bigger version of the normal Jellyfish. They drop more Glowing Jelly than regular Jellyfish. The player can stand on them (maybe they have the same effect as the note blocks in Mario, where they sink a little bit).
  They can be obtained with a net and placed again in water.
• Fish
  It's a swamp... we need more fish. They can come with different textures. They flee when a player is too close. They are hunted by Lurkers and Anglers. They drop fish meat. (maybe for when we get to Fishing?)
• Sporeking
  I just really like the pun here. But really, it could be a little king of Sporelings, which could provide you with some rare spores or whatever.

Farming

• Silt Crab Gargantuan
  Sort of a mini boss. This rare mob is a big Silt Crab, which is quite hostile. It has quite some damage, and drops a lot of crabclaws and even pre-made crab sticks. However, you can lure/tame it into captivity with Angler Meat.
  This might be useful for a Silt Crab farm, as it lays eggs over time.
• Nautilus
  Somewhat rare, aquatic farm animals that can be caught in a net. They would drop their meat (maybe calamari? ;)) and their unique shell, just like snails. The postive side is that they lay multiple eggs per time, the negative is that they are rarer than snails and can't be out of the water for too long (so when you catch one, you'd need to hurry or it dies).

Hostile

• Gigantick
  A huge tick that roams the woods, it has a bunch of eggs inside of it, so don't let it suck your blood, or some might hatch. (No drops yet)
• Fluvyad
  An aquatic Pyrad (Fluvial + Dryad). Their looks and (mainly) A.I. will be different, but they belong to the dryad family. I was thinking these could be found in a deep/flooded forest biome. Perhaps they could also have unique tools/weapons like the Pyrads.
• Angler variant (proper name needed)
  I think it would be nice to have a bigger variant of the Angler, It wouldn't spawn in groups, and actually hide and wait for prey to swim by.

Neutral

• Dryad
  Neutral nature-based mobs. They spawn in forest biomes but are still quite rare. Maybe these would be the most powerful and pure of the dryadic family. Since they are neutral they would only attack if being attack themselves, but they also hate to see a player chopping trees or creating fire.
  They could drop some rare loot and actually be mini-bosses.

Utility

• Scout
  Craft a Stalker Eye with 4 Dragonfly Wings to create an inanimate Scout. When animated, this utility mob can be accessed by the radial menu. This will allow the player to take control of it, kind of like a flying camera. Because of the Stalker Eye it has a good vision in caves, making it perfect to scout ahead. However, mob will still be able to attack you or the Scout, so be careful. When a Scout is killed, it drops an inanimate Scout.

Flora:

• Deeplake Lily
  A lily-like plant which grows in the cave lakes. It floats on the surface and has tiny roots which obtain water directly from the waterbody, instead of growing on the bottom.
  Deeplake lillies grow near life crystals, making them good indicators for the rare mineral.
  They could emit insect particles (cave moths anyone?).
• Models for bladderwort
  Just thought they might look better with models
• Ferns
  FERNS! We don't have ferns, even thought they are beautiful plants. I'd probably moan about making the model though, but I know it'll be worth it.

Blocks

Building Blocks:

• Rubber Blocks
  Storage blocks for rubber, they could have some sort of bouncy effect like the slime blocks in Vanilla.
• Scabyst Blocks + Bricks
  Not really a suggestion, they just haven't been implemented yet (textures were made a while ago).
  Although we could do with more variations of them, as the colour is pretty nice.
• Amate Paper Blocks/Walls
  Would make for some fancy oriental themed building blocks that could fit quite well with the swamp. And of course, they could function as storage blocks as well.

Mechanical Blocks:

• Amate Paper Doors
  With Amate Paper Walls come fancy sliding Amate Paper Doors.
• Rope Door
  I don't know the official name of these, but you see them a lot in warmer climates. Basically a bunch of fancy ropes in your doorway, you can walk straight through (probably requires a nice model). Mobs can enter, but they do not see you when you are on the other side. So they are mainly for decorational purposes.
• Animal holes
  Just like rabbit holes (except we don't have rabbits). They can generate on the sides of dirt (or dirt-like) blocks. Some critters might use these to flee to. Traps could be set up here maybe?

Items

• Phantom Bow (Needs new name)
  Could be a somewhat rare loot item. It shoots a bit slower than a Weedwood Bow, but besides the arrow you shoot, it shoots 4 extra phantom arrows in slightly different angles (they spread). They do less damage than the arrow you shoot, and canot be obtained, but might still be useful for lazy aimers, or hordes of enemies.

Other

• Particle effects in caves
  Just some other insects or falling dust to increase the atmosphere a bit, and make the caverns feel a bit more alive.
• Gentle rain event
  As we have the Heavy Rain event, I think it would be nice to have a more peaceful variant. A gentle rain with a slight bit of fog in the distance. Maybe it would have a chance to be followed up by Heavy Rain.
• Custom rain sounds
  The Vanilla rain sounds aren't really pleasant to listen to. And if we add two types of rain, there should be a bit of a difference in the sound effect.
• Dark Woods
  What is this, a biome with landmass?! Yes, shut up. The trees wouldn't have wide trunks and wouldn't be too tall either (probably about 8 blocks until the canopy?). Yet they grow quite close to eachother and with quite a broad canopy, they hide the sky. The terrain would be slightly hilly, with some eroded paths. The ground is mainly covered with ferns and some bushes. There would be an occasional (tar/stagnant) pool. Maybe actual Dryads could spawn here.
• Deep/Flooded Forests
  A forest that is entirely underwater, yet still seems to flourish, Besides the trees there would be big, overgrown roots and occasional cragrock formations/hills. Althought the water is quite clear, there seems to be a darker layer covering the surface, allowing little light from the skies to come in. Instead most light here comes from some plants and mobs. Fluvyads really feel at home here.

Yes.

Yes?

Yup.

Indeed.

Mhm.

Updated with:

• Gigantick (again, a great pun)
• Fluvyad
• Angler variant
• Nautilus
• Dryad
• Ferns
• Dark Woods
• Deep/Flooded Forests
• Animal Holes

"Animal Holes"

Beggin your pardon?

( ͡° ͜ʖ ͡° )

But yeah, call it what you want, I can't be bothered translating it correctly now. Haven't had coffe yet :/