What made the earlier games special is that they took us to  old worlds that hadn’t really been explored (much) in other games and which were quite different to what most people are used to.  

If Ubisoft don’t think there is enough space left in the timeline then perhaps the best option would be to look at different geography.  I’ve haven’t played Revelations yet so there’s parts of the series I haven’t seen yet, but I believe that so far all the games have been set in a European/Middle Eastern setting.What could work is a parallel game in an oriental setting, set for example in the Ming Dynasty China.  That would keep the historical feel and still allow the assassin to operate in a similar fashion, but open up avenues for new landscapes, designs, items etc.