We get a fair number of people who join the game and say, “Oh it only has 4-5 enemies, it sucks. Minecraft has more.” Which is ridiculous, obviously, because most of Minecraft’s enemies are just re-skins.
Sheep, pigs, chickens, for instance, all bare minimum programming, all just simple skins. Many of the enemies have only one attack, but the total enemies are varied and numerous.
The illusion of numerous enemies and neutral enemies is needed for immersion.
Worlds should have non-aggressive enemies with simple skins and AI. Some simple birds that squawk, hop in and out of trees, some simple frogs in the marshes/mud areas, camels in the desert, lanky deer in forests, cows in the plains, etc. Give them ingredient drops and weaker food bonuses. Allow people with max stealth to approach them more easily.
edit In Minecraft, Zombies, Zombie Pigmen, Villagers, Ravagers, all look nearly identical and have rudimentary animations and AI, and yet, people consider them different NPCs.
Regarding Boundless enemies, some T5+ Wildstock could be changed to “Rhinoceros” and be given the ability to bulldoze 2 columns of blocks.
The T5+ or so Spitters who gain the long-range bombardment ability could be called “Snipers”, their appearances changed a little, and mixed in with other Spitters.
Some Cuttletrunks could be changed to “Pterodactyls”, and given a simple swooping AI.
Some Roadrunners could be changed to “Flamengos”, and given the ability to create lava anywhere they run.
Minimal effort, so new players see a world filled with creatures and farming, instead of seeing the word “Wildstock” over and over again for 100’s of hours.
Sometimes quantity is more important than quality.
The key is to make minimal cosmetic changes to existing models and altering their name while adding a new ability as well as creating some very simple AI neutral creatures to populate the world with.