If I'm a god with all the time in the world to do it?
To start with, we gotta have spontaneously generating monsters with varying levels of aesthetic coherence and a limited but large selection of random characteristics based on where they generate.
After that, humanoid and intelligent versions of those creatures, and some sort of mechanic to let spontaneously generated lesser creatures become greater creatures. Probably whatever I think will make the world most interesting to observe from afar or try my hand at living in.
Then we need humans, gotta have basic bitches.
That implies I need to figure out what the 'base monster' form of a human is. Something to keep in mind.