There are dozens of great battles of history that could be utilized.
As an idea how about doing a years worth of theme. If you are doing 6 scenarios this year have them all themed around the same thing....to use the old meme of WWII you could start with the Normandy Invasion and through the year work your way to Berlin sort of thing....
You could even actually have the scenario dates set...but depending on the outcome of the first one, you determine the theme of the second one....Say for example your first one is the Normandy Invasion....if the allied forces get beat than instead Battle of the Bulge, you do a new fictional invasion, say the Invasion of Italy or something like that. So the entire year is centered around WWII and potentially re-writing history.