I wonder if it would be possible to make a Half-Life 1 themed mod that added in both Xen and the various creatures. I kinda wanted to make it for myself at somepoint, but I don't have too much freetime so I'll put the ideas here.

I think getting jumped by various Xen wildlife as a slugcat would be funny.

List of funny creatures:

Headcrabs, funny little things that attach to your head and make you into a headcrab zombie. I don't know if an infection system is possible on rainworld but headcrabbed creatures would be funny.

Gonarch, big brood mother that spawns tiny baby headcrabs.

Houndeyes, funny three legged shockwave producing dogs that go in packs. The more houndeyes there are the more damage the shockwave does, might be possilbe in base rainworld stuff.

Bullsquid, two legged creatures with squid tentacles near their mouth that spit acid (i think its acid). They hunt headcrabs and other things, usually in pairs.

Tentacle, big green tentacle with red eyes and a giant knife for a nose that detects prey using sound. Would work kinda like a pole plant or monster kelp but it would stab at you or attempt to grab you.

Hyrda, a translucent blue tentacle with a spike on the end. It is blind and attempts to impale creatures making too much noise to eat them. So a scarier pole plant.

Antlions, blind creatures that use pheromones and vibrations to track prey usually in very large colonies.

Boids, bird-like creatures that fly around in flocks could hunt batflies or such but be neutral to the player.

Chumtoad, kind of a meme creature but it would be a neutral frog-like creature. Could be eaten like a batfly maybe.

Protozoan, kind of just a floating single celled organism that doesnt do much except float about.

Snark, panicky landmines, they come from snark nests and go for the nearest target to nibble on them and then explode.

Barnacle, attaches to the ceiling waiting for something to touch it's tongue before pulling it up. Spits out inedible items.

Icthyosaur, big fish that wants to eat you.

List of Environmental ideas:

Healing Pool, could give a bit of food per day or bring dead creatures back to life slowly.

Plant Light Stalks, a stalk with a light at the end that retracts when something is near.

Xen Tree, a 'tree' that whacks you for getting close.

Xen Trampoline, a growth that propels you up when you jump on it.

Xen Fungus, spore things, could emit a spore cloud on damage or contain the one puff item.