I think land mines and other hampering devices for hampering attacking militaries are a vital part of modern warfare that is not present in this game. So here's my latest suggestion:
Land mines--- Buildings that can be built in provinces and cities that damage any enemy units there, and if it's in a city, it has a slow degrading factor on population. Mines can be defused for an additional price, but only by the player that owns the mines. A special unit can remove them regardless of nationality.
Anti-tank mines---Same as land mines, but do much less damage to infantry and much more for tanks and other vehicles.
Land mines--- Buildings that can be built in provinces and cities that damage any enemy units there, and if it's in a city, it has a slow degrading factor on population. Mines can be defused for an additional price, but only by the player that owns the mines. A special unit can remove them regardless of nationality.
Anti-tank mines---Same as land mines, but do much less damage to infantry and much more for tanks and other vehicles.