I'm cool either way, and I'm cool with doubling (or even tripling, quadrupling) up and adding NPC-type side characters. And they don't have to fit! Clashing characters are even more fun. 

We can share plot ideas here or PM, let me know. As I mentioned, I had a good one for Gilgamesh's master that could probably be adapted to OCs, though I don't know my ancient myths and legends as well as the creators of Fate do.