The one major problem that I see with a futuristic setting (even a modern setting), is how Ubisoft can possibly implement modern weapons into the Assassin's Creed universe, without completely overhauling the current game-play mechanics of the series. I always remember the end of Assassin's Creed 2, when Desmond was slitting throats with a police baton, and waving it around like a sword
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So, I think Assassin's Creed will have to remain in the past; preferably at a time when swords were still being widely used, and when gun technology was still in its infancy. A proper futuristic or modern setting just isn't possible for where the series is right now, and Ubisoft doesn't seem to be interested in dedicating a lot of time toward development, as they're releasing a new Assassin's Creed game every year.