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.