Yeah, I think early to mid 1800's is about as far forward as they can take the series. They've been advancing time over the course of the series, so maybe the ending of 3 will give us a reason to reset the clock. I think they could do a lot in Old/New Kingdom Egypt and Asia throughout most of its history.
They made pretty explicit nods towards 8 potential prior time periods in AC2, so I wouldn't be surprised to see them take a detour towards those historical assassins they had statues of.