Hello! I don't know if this topic has been brought out before, but I thought the faction idea in Mirage was really great and i'd like to see them in other worlds.
One example could be the humble goat monks or the honorable ronin of Mooshu. They could each sell a unique dorm-like housing item. A badge representing your nobility with either faction. And special cosmetic or crafting items.
Another example could be the 3 different races of manders in Krokotopia. They would each give a fire, ice, or storm themed dorm, a badge and special cosmetic/crafting items.
Adding factions to earlier worlds not only gives more content do while completing it, but also gives good endgame content for people already max.
Another side idea for factions is a window in your friends list to view all the factions they have become initiated into and completed.