Two modifications I thought about are attached artillery and reactions.
The idea with the first one is that activating attached artillery diminishes the rule that only the same type of units may be activated on one command.
Attached artillery - I suspect it would be only one base - may be activated alongside infantry and cavalry formations, and of course should shoot, move and reload regardless of what the other units in the group are doing.
This would give a more integral feeling to the game, instead of having to activate detached batteries and sacrificing the momentum of the infantry and cavalry - artillery should be used as an offensive weapon, and not something to pelt the enemy with from a safe distance.
By extension, this also means that artillery should not be completely defenseless against melee attack - I'm still thinking about an elegant solution, as it should also be the duty of the units in the brigade/division to keep their guns safe.
Giving the players a chance to react means taking control of a risky situation by sacrificing their available Command pool for their coming turn. One die or command card may be played immediately to react to enemy advance or shooting. The idea is that the crisis has gained the commander-in-chief's attention, while the rest of the field is ignored - a card or die that could otherwise activate two or three units is played to control a single one. This may be limited, so as not to tamper with the IGOUGO system too much, to one, two, or half of the available Command dice or cards.
It is an automatic success if we use dice activation, unless the unit is untrained, in which case it must pass the command test. For example, if the opponent declares a cavalry charge, a reliable infantry formation may form square instead of waiting to be caught in the flank.
Reacting to enemy fire with counter-fire means that the effects are applied simultaneously, and both units could rout as a result.
This, as a movement (re)action, could potentially allow shaken units to retire before they are destroyed, with the added difficulty that they must show their backs to the enemy.
I may also be able to bridge the two points together, and state that an artillery unit that's charged may fire canister as a reaction (if the guns are loaded).
By the way, I'm steadily increasing my painted Napoleonic forces, and if the summer heat allows, I'll be able to play a game this weekend.