Ancient battles are linear in nature, just a lot more compact than ACW.
The real problem with an ancients or medieval system is coding a huge array of new troop types and modeling their interactions, it would be a whole new game
Other mid-19th century wars would be the easiest port, although European cavalry would have to be modeled differently, which isn't too much of a task I guess
Napoleonic would require grappling with a lot of new code for troop types and interactions not represented currently - infantry brigade formations (line, mixed, column, square, all the above with skirmish screens), cavalry with different roles to ACW cavalry (cuirassiers, dragoons, hussars, lancers)