I will run au GF:firefight game during a convention this week-end and I will uses these optional rules I'de like to share with you to get your advice. Have to say I have already used them and I think they work quite well!
When a unit is activated, if it uses the "hold" action, it may do one the 3 actions below: 1) to the ground: the unit can't shoot this turn but gain an additional -1 to be hit cover bonus. 2) aimed fire: the unit's ranged attacks all gain an additional +1AP 3) overwatch: the unit can't shoot now but it can interrupt an enemy unit's move to shoot at it later. Use a marker to remember this state. Overwatch is lost if not used when the unit is reactivated.