well it's a new game in, as they state it, "a new era". They should have called it a new decade though, since in my opinion the 1720' and the 1770' still are more or less the same era. But yes this could have been an exp pack rather then a new game. Also how long has it been since ACIII was released ? 4 months ? looks like ubisoft is going to exploit the new engine. Just spam as many titles as you can, the stupid customer will buy anyway.
And how very convenient to stay in the 18th century. just port everything from ACIII make a few new skins and models, done new game.