I wouldn't mind different branches such as Warrior of Light and Warrior of Darkness. For example, you could be a WoL Paladin or a WoD Paladin. It would help with the whole "balance" theme and all that.

Perhaps there would be a difference in animations for certain spells/skills or even specific abilities that either a WoL or WoD would get (like role types or limit breaks). Also, there could be changes in the story, specific quests you could undertake, and maybe even specific weapons/armor you could obtain depending on your branch. Due to all of this, it would also not be a decision that is easy to reverse (i.e. it would not be as simple as swapping out your weapon).