Three Tenants of the Creed.
Stay your blade from the flesh of an innocent
Hide in plain sight
Never compromise the Brotherhood
The great ironies of the Assassins.
The Assassins seek to promote peace, but commit murder.
The Assassins seek to open the minds of men, but...
Well just think of two locations the players will go, what they will face, and what will be in there way. Once you do that. You probably have enough time to think the rest up before they get there.
Alright. Then, you have the ground, and need to build upon it. What places will they go to, what places will be available? Takes a lot of thinking. But, I never had the pleasure of playing Final Fantasy 5 and up. So, I'm clueless on some of it unfortunately.
Well the first thing you gotta figure out is if you want pure a pure OC RP. Or OC's with Canon people. Then, you have to figure out what setting it would be in. In this case Final Fantasy, so the question is where in this universe? Can always make it up as you go along. But, it's best to have...
Very intriguing I'd like to know more about the plot and what you have in mind. While it sounds like fun. I do want to know what I'm getting myself into.