The Assassin's Creed series has followed a fairly familiar pattern of jumping forward a few hundred years with each numbered installment. Now that Assassin's Creed 3 is hitting the American Revolution, that pattern leaves a relatively short window before we hit modern day. Ubisoft assures, though, that future games may go back in time to earlier points in history.
"It's a question we've been getting a lot," producer Francois Pelland told Metro (via OXM). "It's not a moving forward thing, it's all based on lineage and on DNA and therefore it has nothing to do with time. The next game could easily go backwards as much as forward, no problem."
Assassin's Creed 3 promises to be the end of Desmond's story, so the next numbered game in the series could introduce a new "modern day" protagonist. Or, Desmond could simply have ancestors from other time periods that weren't relevant to the overall plot before. Given how varied the possible locales are for the franchise, it's clear that Ubisoft will keep the franchise going on for a long time.