To begin with, you will need a larger playing surface. TOI boards seem to be well adapted and can be purchased separated from the boxed set.
In theory, the command structure modeled by the rules should be adaptable to a multi-corp battle (Waterloo?) but with the addition of a third tier of command (the Army Commander). In the large Napoleonic battles, the corp commanders were given objective orders such as "capture Hougoument" or "hold this ridge line". Whereas the present scenarios work well with operational orders (attack, defend, maneuver, reserve) the army commander would provide strategic order such those mentioned (objectives). The corp commanders would then execute using operational orders. It should be easier to change the orders to a corp commander but take time based on the distance from the Army Commander(unless he rides there himself (or has the corp commander with his leadership command radius.
Next on the list is army level units. The Grand Battery comes to mind.
Grand Battery: Any artillery available to the army can be formed into a group by the army commander. The max batteries within the group would be equal to the army commander's leadership value. This group would be in a line and would fire as group (each battery chooses its own target).
Reserves are usually under the direct control of the Army Commander.
Armies lived and died by the overall morale. We need a mechanism to account for and then test the overall morale of the army, probable related to objectives lost and units lost or routed.
These are some quick swags.