I know they’re in different spots but:
For undead Undead legion of the frozen catacombs (and more!) is pretty filled out.
I’m working on Vampires monsters whenever I get time/need more vampires, unfortunately it is progressing slow.
Kobold, Goblin, Bandit and other assorted monsters. Johnny Blood's Creatures and NPCs
I also did some spiders awhile ago… Giant Spiders Mobs
Gargoyle thing, ooze, dust specter and a giant skeleton, High Level Fantasy Creatures
These look interesting, and unlike the name suggests, they are monsters, not a boss The Dream Weavers
I tried to lay this out so you could maybe have your GMs use it as a starting point and find everything? (so the monsters are for the most part in nice large chunks)
It’s not everything on the site but it should be enough to do a campaign with. There are some really awesome bosses on the site too for your GMs to check out but I didn’t link them because they’re a bit less versatile.
Your group could also use the NPCs other people have made and a few of your own and make an Open Legend Monster Manuel. I bet most people here would like that.