Assassin's creed is one of the best gaming titles by ubisoft and obviously the most loved one (except assassin's creed 3). If ubisoft has decided to make a movie on creed, they should definitely consider having story from one of their game series in it. I can back this up with an incident. Some time ago Movie Need for speed was released, so me and my friends went to watch it and after when it was over all I heard was "Wth!! Movie was nothing like any of the game at all" and we were disappointed and so were many other who have some interest in gaming. So in order to make this movie a success ubisoft should definitely go with one of their game's story line.
I think Black flag and story ahead of black flag featuring Edward Kenway, Haytham kenway, Connor kenway and The one from Rogue - Shay Cormac Is the way to go.
This will include American revolution from assassins creed 3, and all the pirates stuff from assassin's creed black flag and rogue.
Also they should consider making movie in parts and release in the same way as they did with these games.