The ability to assign generals to lead specific armies. For example you could assign a general to the 1st army to defend your provinces, and assign a general to the 2nd army to capture all of the enemy provinces,etc. Could also have air marshals and assign bombers and fighters to go to a specific area.
It would be nice if a demilitarization mechanic would be added to the game. A few possible options for demilitarization would be:
National Demilitarization: In a specified province, only a certain/specified country is not allowed to put troops on it. (Ex: Demilitarization of the Rhineland)
International Demilitarization: In a specified province, no country is allowed to put troops on it (Ex: Antarctic Treaty prohibits any nation from colonizing it, so preventing troops entering would prevent it from being colonized)
Partial Demilitarization: In a specified province, a specified number of troops are allowed to be in it at a time (Ex: As a result of the Yom Kippur War, Egypt was limited a number of troops that can be put in the Sinai Peninsula.)
It would be even better if it would be possible to make demilitarization a possible option of a peace treaty, and a remilitarization option for events.