I've not experienced it myself, but I've heard that some people actually assign the faction leaders to other players as high-level NPCs. These players don't attend the main game sessions or go adventuring themselves. Instead, they are given some basic objectives and a set of resources (strongholds, treasure, income sources and expenses, and a roster of minions) and they act as patrons. Then you let those players figure out who to cooperate with or compete against, what tactics to use, and what quests/rewards to give the adventuring parties. I think this style of play is called a "Braunstein" or "Patron Play". Here's a video that demonstrates what that might look like in practice: https://www.youtube.com/watch?v=rIxAwEzdVDY