I personally would like assassin's creed set in Byzantine Empire or Roman Empire.I would like to see ac set in Byzantine-Bulgarian wars,meet Basil the Bulgar-slayer(no offense to my Bulgarian friends) or the war between Rus and Byzantines.Roman Empire would be perfect for an ac game because of all the drama between the politics,assasinations,wars, the civilization and much more stuff!
I'd like something that blows me away as much as Origins did. Didn't like Odyssey as much, too many details were cut from it, and haven't played Valhalla, but it looks similar.
My dream would be a complete remaster of Black Flag with the new game mechanics and graphics engine. Or something set in Mediaeval Asia, something with ninjas or samurai.