Assassin's Creed: Odyssey - 9 Future Settings Ubisoft Must Use
4. Europe In World War II
There are already too many games focusing on the front line of World War 2, but few look at the home fronts. Suspicion and antagonism was rife - everything thought their neighbour was a spy, or was robbing their house when the bomb sirens went off.
This would make a great setting for a game in which you actually were a spy, especially if the game took place in Germany at the time, and the Assassin was a spy for the Allies. Staying out of sight of Nazis and normal citizens would take equal precedent.
A game has done a similar thing - in the fantastic Le Saboteur you played a member of the French Resistance, with focus on free running and stealth, but Assassin's Creed could easily work out their own take on the famous conflict.
An additional benefit is that the games wouldn't need The Animus to explore the setting, as it's in living memory for some people. Jumping in and out of the Animus is the most annoying part of the series, but if the game was purely based on flashbacks by someone's grandpa, Ubisoft could do away with that annoying story feature.