A mechanic for the German Confederation and the Swiss Federation pre Sonderbundskrieg would be great.
Futhermore, I think a VicIII could really profit of a mechanic to simulate federally organised systems better Vs unitary states like France. THis would add so much flavor not only to Germany, Austria (KuK) and the US but also to the states in Southern America with the conflict between unitarism and federalism going on.
A slow shift from a post-napoleonic to a modern military should also be simulated. This could look similar to HoI3 but having the command distance so short as to have the units move as a stack. Of course, if you have, say, a capable corps commander, moving a corps on its own shouldn't prove too much trouble.
Additionally, battles could be joined piece by piece, as it was not unusual to have units need hours or even days to reach a battlefield. Here, railways could come into play: The better the infrastructure of a province, the faster units can move to join the actual fight.