For the gameplay : I understand that full-scale wars and year-long campaigns can be difficult to implement. Then why not adding instead a bigger map where we can move our regiments, divisions, on a larger scale (days & tens of km) as the map that currently shows the mission briefing ? This would help personalize the battle a step further. Then, when they clash, we zoom-in to the current battle scale (hours & km). "Kind of" total war, but still for one battle only.
For the era : The wars of the league of Cambrai & Cognac : a melting pot between Milan, Florence, Venice, the Papal States, Spain, the HRE and France fighting for the wealth of Northern Italy.
And, obviously, the battle of Ligny, Qattre-bras and eventually Waterloo. It would fit this game perfectly.
Every Napoleonic battle would be great actually, Austerlitz, Dresden, etc... each of them being a DLC ?
EDIT : this link is a goldmine should the developers decide to do the Waterloo 4 days campaign ! http://en.wikipedia.org/wiki/Order_of_battle_of_the_Waterloo_Campaign