Please do not make the favor system a reality.
I have to say that I really do not want this system, it seems like a waste of development resources considering where we are right now. So many more things could use work or additions before a fluff system that allows people to creep on NPCs. I know that's not a very fun request, I just feel like this would be a really awkward, really niche system that the game just does not need.
For more system design, lets see things like more effort into personal housing, the Gold Saucer or my most wanted feature: gear customization. This is the only MMO... maybe the only modern RPG on the market at all that has no way to customize your gear at all, which is agonizing with a hard accuracy cap.