Primitive Cave

After defeating Urschleim, the player could find a key in a ruin that opens a portal to a nearby cave in deeper layers, or to another region of the world—the primitive caves where there would be prehistoric creatures similar to dinosaurs, insects and arachnids, and marine animals.
There would be 3 main biomes: Mesozoic, Cenozoic, and a marine biome with their subbiomes, bosses, creatures, a new variant of cavelings, pets, weapons and armor, decorative items and tools, and perhaps even "dinosaur" mounts.
It would be interesting to have bosses like a giant radioactive spider, a lava T-Rex, and an albino megalodon with bioluminescence.
Mini-bosses like green slime and its little mini-slimes (whose ability would trap you in goo), the queen ant in its anthill subbiome similar to the larvae in the main cave, the emperor scorpion in the bone desert subbiome, and a mosasaur in the sea.
We could find mounts that would allow us to move faster, similar to cars but with more control.