Not sure why people are set on two factions and a hard line between them.
The secret world had three factions that were suppose to look at opposing each other for whatever reason. For the background and story, it was kinda this way. BUT gameplay-wise you could party, raid, quest, talk, etc with everyone.
WOW could go this route. Where story and lore there is a direct split. But us being adventurers, heroes, and saviors of the world, galaxy, universe, etc. WE get to choose who to party with, quest, seek help, raid with (AKA everyone in the wow playerbase).