*The overthrow of the Tsar Empire by the Communists during the Russian Revolution
That could be interesting, but I'd be wary as the Assassin's are supportive to communism (Lenin and his brother) and the commie Russia shouldn't be portrayed as the paradise on Earth. And Lenin was a damn murderer(Ukraine; invasion on Europe; hell, execution of whole Romanov family) as well as Stalin, so don't spout that "Lenin was the good guy" shit.
*The rise of Ceasar's Empire in Ancient Rome
Pretty interesting, though again: I wouldn't pleasantly see Julius portrayed as a bad Templar. People loved him, after all.
How about renaissance/baroque Poland-Lithuania though? The reign of Sigismund III Vasa would be perfect for AC's storyline. We have an elected king (Templars rigged the election!), who's a devout Catholic (or so he says... Templar) his advisor is a jesuite Piotr(Peter) Skarga (Templar?!). Also, the 'rokosz' (legal rebellion against king in Commonwealth of Both Nations) - Assassin's try to seize power from king, of course - later portrayed by Templar's as greedy and self-centered people; taking Moscow - Assassin's may try to incite rebellion there...
Also, Poland-Lithuania is portrayed near to nowhere, but so is Russia.
They can do anything, as long as they have new, good ideas. I hope they won't jump on the CoD bandwagon.