According to an interview over at Eurogamer.net, it appears that Assassin's Creed franchise actually has an ending that is already written that will be used once Ubisoft decides to use it.
While speaking in an interview with Eurogamer.net, Assassin's Creed game director Ashraf Ismail noted that a rough outline of the ending narrative is already written and Ubisoft will implement it when the right time comes. Ashraf Ismail noted that "We (Ubisoft) have an idea of where the end is, what the end is." Ismail also confirmed previous statement made by Ubisoft's CEO Yves Gullemont that Assassin's Creed is an annual title and that although there is a general plan, there is still room for changes if necessary.
Since Ubisoft works on multiple Assassin's Creed titles, including three more right now, development teams have an ability to implement hints between titles. “We’re now able to seed stuff earlier and earlier in our games. So for example in Assassin’s Creed 4 we have Edward, who was seeded in Assassin’s Creed 3. But there’s a lot more stuff in our game that is hinting at other possibilities,” Ismail said.
The latests title in the Assassin's Creed franchise, the Assassin's Creed: Black Flag is scheduled to launch sometimes at the end of October or early November for PC, PlayStation 3, PlayStation 4, Wii U, Xbox 360 and Xbox One. Of course, next-gen will be available as soon as Microsoft and Sony decide to launch their own next-gen consoles.
Source:
Eurogamer.net.