The era of pike and shot.
Although a set of ACW campaigns around the Fredericksburg and Chancellorsville area (using one big shared map) would be great.
I realize that this will be a contrary opinion from most people's but I'd rather you don't go that route. As Sid Meier says in his Covert Action Rule "One Good Game is Better than Two Great Ones".
http://www.designer-notes.com/?p=119