Considering all the different eras and places Assassin's Creed has been to, why not the stone age? I for one would be very interested in wielding the "bonus DLC weapon" shown at the end.
Or just a Flintstones crossover where you get to assassinate Fred and Barney. I'd be down for that too. Just take a lesson unlearned from the other games, and make sure there are cavewomen too.