Revelations is the end of the Enzio and Altier stroy arch. Its not the end of the main story involving Desmind. From what I can tell based on what I have played by the end of Revelations Desmind will be good enough at moving and thinking like an assassin that they will probably move forward with his story, or they will put one more assassin in a more modern period that you will play through.
Ubisoft has gone on record stating that there is no Assassin's Creed 3 yet for a reason and that some new plots and characters will be revealed when they finally go to 3. Think of this like San Andreas for GTA. Its just the uber final curtain for all the game assets they current have to pump out a sequel per year.