I think Ubi moved the goalposts after they recognized the overwhelmingly warm reception that AC2 received. I think it was meant to be a proper 3-game trilogy - the reason Altiar has only one game dedicated to him. Then Ezio came along (with his nifty new toys, superior mechanics and varied missions) and they realized that they could expand on the idea to include more of the historical elements that they had wished for.
My view - AC (Altiar) is a short prolog, AC2 (folding in Brotherhood and now Revelations - making a trilogy-within-a-trilogy) will be Ezio's chapter (and if you've seen some of the E3 footage you know there is a bit of this mentality going on).
After that I have to speculate, if Ubi wishes to stretch it out they could introduce a new ancestor and spin that into another trilogy, but honestly after AC:R I see three more games, max, to give us Desmond's story.
Heck they could even do a post-script game, it would make a nice symmetry.
1 Altiar, 3 Ezio, 3 Desmond & 1 Adam (?)