I think it will be new game. 2 years is what they've been going with so far.
They might release another story DLC in the meantime, so that season pass might actually be worthwhile.
I like this direction they're going with. I was wondering when they were going to start addressing the height of the Greek and Roman empires. If the next game is set in Greece, think that inevitably means the next one after that would be set in Rome, perhaps around the time of Julius Ceasar. Rome around that time would be
This new direction is great. I really disliked Unity and Syndicate. Both felt like the same bland game with different wallpaper.
Right now I rank Black Flag as #1, Assassins Creed 2 (mainly the first two in the series) #2, Origins #3 and the original #4. The rest were just so so.