One of the games I a have been running a little and working on a lot started me thinking on what I hate about shows like Grimm and X Files: The monster of the week. A world with hundreds(maybe thousands) of monsters does not make a lot of sense. True we have to deal with a weekly or so game, and we have a lot of encounters to fill up, but a limited monster game could be cool too.
When I started to world build I limited myself to 10 types of PC killers, but most of which have ten or more racial or regional variants. One of the main ones in the setting is werewolf there are tons of movie types to draw from. Another is variants in Wight to include some that are Liche like. The Apex predator in my word is the chimera, I have tons of heads, bodies and magical types( this has replaced the dragon in the myths).
Make sure that you fill all the niches that you will need when you start, you also might want to leave room for a cool future idea.