Although I'd like to see the series return to the defining premise of, well, the assassin's creed, there's still wiggle room to include some of the ideas implemented in the later games.
I think Ubisoft is not clear on where they want to go.
I'll go out on a limb and say that AC1 was a mostly driven by Desillets' vision, after the backlash, he refined the whole thing and launched a more conventional AC2 (with a setting that he though seemed still very passionate about, going by the interviews).
Brohood was created with the scraps of AC2, and Revelation was an up-port from a portable game, just to fill in the gap before ACIII.
ACIII is the first, real, full game of the series without the footprint of Desillets, for better or worse, and i think Ubisoft as found themselves with a huge brand on their hands, the gamedesign of which they don't fully understand, so they try to juggle all this elements while trying to appeal to more people possible.
Results may vary, but i don't think they intend to go back to what Desillets had in mind, and i don't think they will; even with Black Flag i think they'll just try to create a thrilling action blockbuster..
thing, while PRing their way through the fans of the more stealth side of it.
With all that said, i still think this brand is one of the most interesting around, if anything for the setting that is not common sci-fi/fantasy/modern/cyberpunk/pickyourpoison.
It has the ability, or should i say the "potential", to always stay fresh and reinvent itself very easily (whether they'll do that or not, it's anyone's guess) so i most definitely do not want it to die out.