My session ended while I was typing apparantly. Here's the fixed verison.
Open to suggestion about a mechanic where coming out to attack has a useful advantage in certain situations.
I have been reading up on this idea and I can see some similarities between a sally action and a boarding action.
There are a few scenarios in which a sally action can prove beneficial:
-During the initial stages of an invasion in which the attacker has not had the opportunity to dig in: combat has not yet become a siege but the defending party could initiate a sally action to disrupt enemy actions, delaying opponents in order to give the defenders more time to dig themselves in. Could be interesting if a planet has to prepare its defenses from a surprise attack. Like with effort to set up the Atlantic Wall, fortifying a planet should be a timely and costly affair. That said, digging out forces from a fortified position requires the attacker to have a larger army.
-When the invasion force has landed and actually managed to surround the defending party, sally actions can be used to achieve a few other objectives as well. One of them is sabotage. Due to the abstract nature of the game a sally action can mean a great many things. Elite troops might even have technology capable of entering the enemy camps undetected through a secret gate, a tunnel system or even teleportation. From my standpoint I do not find it impossible for the defenders to be able to reach the attackers siege equipment or a forward command post. The overall damage of a sally action might by comparatively minor but when successful should provide a proportionally large hit on enemy morale. Another reason to sally forth is to act as a distraction for allied forces; a costly affair for the defenders but when the defenders have allies outside a besieged strongpoint, drawing in attacker could give their allies the opportunity to launch an attack against the attacker's flank or rear.
-Another scenario consists of two 'equal' forces. Each party has managed to dig themselves in and neither party has the strength to dislodge the other from their respective fortifications. The role of defender and attacker switches constantly. Think WW I trench warfare. Sally action in this situation can range from sabotage missions, all the way up to mass charges across open grounds. Sabotage missions deal little damage, but affect morale, while charges provides a change to break through. , but the attacker risks losing many troops in the process, while the defenders risk a breakdown of moral; their troops panicking and abandoning their defenses.
It might also be an idea to add special strategic actions to commanders, granting benefits to forces with experienced commanders.
I have to say that I like the changes to the army structure and I wonder if a similar abstract system can be implemented on planets: depending on the planet's size and available resources you build a number of nameable slots, which you can fill with structures like factories or farms. Perhaps with enough time, resources and technology you can keep on building, increasing the size of a planet, changing it into a superstructure. I imagine an empire stripping entire systems to expand their home planet.