World war one would be very interesting. Then they would probably build some sort of assasins vs templars cospiracy around Gavrilo Princip (the guy who assassinated the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand of Austria ,and thus indirectly kick-started ww 1 ), maybe even make him a descendant of Ezio. I mean, it's not that far from Italy to the Balkan...
Anyway, it would be cool if it took place during ww 1 as, it has been underrepresented for way too long. Even though it has a lot of possibilities: the dawn of flight, Nikolaï Tesla still being alive(wich is allways a good thing), gas warfare, scientific discoveries...
*edit* After reading the rest of the post, I must say that victorian England would be very awesome too.