A modern-era Assassin's Creed would be alright... but only if using guns is limited, I think. It could totally work though... Remember Hitman?
Really, I was hoping for the French Revolution and the Napoleonic aftermath. Napoleon I's a fun guy, whether fighting for or against him. It would be even better if I could have a mobile Guillotine to carry out my assassinations.
The American Civil would be fun for me if I was supporting the Confederates. Killing an evil Lincoln would be pretty funny, and an added bonus would be turning on General Lee.
But what about the Eastern World? There's got to be plenty of conflict and intrigue during Japan's Warring States period.