​The possibility of placing units on alert which would wake up when a unit from another civilization is in its field of vision. This would allow you not to realize at the last moment when it is too late that the enemy army is at your doorstep.

To go further we can have military units system with :

Reserve Army : dormant units with a maintenance cost bonus and an extra turn to be awakened

Active army : regular maintenance cost regular move

Army on Alert : extra maintenance cost and alert mode which would wake up when a unit from another civilization is in its field of vision

This very simple mechanic, i think, can bring a lot on the military level and unit management.