ArcheAge has another option: make your own! XLGAMES is prepping a July update that includes the ability to create player factions.
How it works is that apparently players who carve out some territory on the northern continent can then create their own independent faction. It looks like there will be a way to establish relations with other factions, tax local estate owners, and get into scuffles with neighbors. Players won't be able to join multiple factions and become dual-citizens, so choose wisely when the time comes!
[Thanks to Dengar for the tip]