Hi! I made a post that I will list every idea that I think would make Subnautica cool. All of the things listed here are my ideas except for the Skeletal Leviathans and the baby animals)
These animals attack without provocation.
Blighted Leviathan (Biter Leviathan Skeleton)
Angler Leviathan (Degasi Survivor Demise Leviathan, Lost River Giant Predator Skeleton)
Super Predator Leviathan (Super Predator Skeleton)
Anemoneye (Host of Clownkoosh, see in passive section. Moves slowly and shoots projectiles like a Tiger Plant. Anemoneye eggs can be found drifting in open water)
Waterfly (Ghost River version of Shuttlebug. Only edible hostile creature)
These animals attack with provocation.
Amphibiornis (No common name yet. Four winged birdlike organism with a semi-elongated reptilian snout similar to that of a dromeosaur. Can "fly" underwater as well as in the air. Back wings act as hind legs. If two or more are hatched from an egg by the player, they will follow the player around and treat the player as a pack leader. If one egg is hatched, no pack is formed. The Amphibiornis still follows the player, though. Immune to Carar, but no enzyme is secreted. If one wild pack member is hurt, the rest of the pack will fight the attacker while the hurt member hides)
Lavabug (Adult Lava Larva. About half the size of a Lava Lizard)
Rockbeetle (Adult Rockgrub. About the size of a Stalker. Very rare)
Cornea (Lava Zone Peeper. Three times the size of an Oculus. Has no pupil in eye, and is blind. Tracks the player by sound when hostile. The secret to avoid detection is to not ride any submarines until the Cornea leaves)
Amphibigallus (No common name yet. Birdlike creature similar to Amphibiornis but taller and with fewer feathers. Tiny front legs with larger hind legs. Not tamable. Same group tactics as Amphibiornis)
These animals will not attack on any occasion.
Light Mesmer (Safe Shallow, Floating Island, and Reefback Mesmer. Lacks the four part mouth fangs and instead has a very small mouth. Will not attack the player. Uses reverse tactics of the Mesmer: It will push hostile creatures and the player backwards and towards the surface. If the player holds an Acid Mushroom, the Light Mesmer will not push the player away. Strangely, the Light Mesmer can walk on land and can be rarely found roaming the Floating Islands. Edible. Can be set loose in a Cyclops or Base, making it an interesting pet. In a base, it will not push the player away. With two or more in a base, the Light Mesmer will breed. It gives live birth. The Light Mesmer limit per Multipurpose room is two. It can be contained by using a hatch, which can now be placed between rooms. It cannot be placed in Alien Containment)
Cloudback (Floating above most areas. Resembles a grayish-white Reefback. The Cloudback sometimes settles on the water surface to rest. On the Cloudback's shell, Skyray nests can be found. Skyray eggs can be obtained from their backs, and can be placed in an Alien Containment [Skyray Alien Containments resemble aviaries])
Sea Dragon Leviathan Baby (Found in Safe Shallows, roughly the size of a Rabbit Ray. Resembles a cute, large-eyed lizard with two tentacles. Sea Dragon Leviathan Babies occasionally follow larger creatures, including players, around to feed off of the leftovers from the larger creature's meals. If a baby follows the player, it can be led to the Active Lava Zone. When the baby arrives, the adult Sea Dragons will not attack the player. Will not age unless in Koosh Zone or Active Lava Zone)
Clownkoosh (Found hiding in Anemoneye appendages in the Koosh Zone. The Clownkoosh is bright orange with white stripes and attempts to hide from the player. If an Anemoneye is killed, the Clownkoosh will dart to another Anemoneye. Egg clusters of Clownkooshes can be found under stationary Anemoneyes. Out of an egg cluster, only three or four Clownkooshes will hatch)
Baby Sea Treader (Found at Sea Treader's path, Safe Shallows, and Crash Zone. Roughly the size of a Spadefish. Has flippers instead of hind legs. Swims similarly to Sea Emperor Baby. Sometimes schools with two other individuals. Eventually, the baby will swim to the Sea Treader's path and settle to the bottom. It will then shed its flippers and will eventually grow larger)
Seadigger (Found in Safe Shallows in the Seadigger Colony. Resembles an ant with appendages attached to its legs. Those appendages help it swim. It also keeps an air bubble attached to its abdomen to help it breathe underwater. Seadigger nuptial flights occur every ingame year at least three times. During this time, Male Seadiggers and Seadigger Queens can be caught. Male Seadiggers can be placed with a Seadigger Queen in alien contaiment/Seadigger formicarium to breed them. The males die after breeding. A colony of Seadiggers can be maintaned with a Seadigger formicarium, which is made at the Fabricator. Can be placed in the Lifepod, Cyclops, or base. The Seadigger formicarium connects to the open ocean. The workers exit and enter the nest at will and will defend the base [and their nest]). Only queen and male Seadiggers are edible)
I'm making the biome list and item list soon. I'll also draw what the creatures look like soon.
I hope you like the list!
If you want me to add one of your ideas, let me know.