What, dryads, fairies, pixies, and the Leanan Sídhe aren't actual myth creatures in your opinion?
There won't be any animal features on any characters in this game, nor will I include monster sex. Every single other adult fantasy game out there features that kind of silliness. Let LLtP be a reprieve from that kind of content. I want things to be fantastical and yet down to earth.
Ya know, it's kinda hard to keep a unified story when you add too many elements to it. Keeping the world-building basic, in my opinion, keeps it moving and keeps it from getting too confusing.
So, humans have a medieval feudal society that was based loosely, knowingly or not, off the fae noble structure. Fomori are humans with a demonic legacy that tried to destroy pretty much everything and everyone.
There's a plot to take power in the human kingdom from the princess. If it succeeds then humanity and fae will be wiped out. The fae and the humans need each other to save themselves.
This is simple enough to follow, complex enough to keep interest and does not overwhelm the guy making this game for us with so much stuff that he never finishes it, dropping it when he stops caring.