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Retreat would be nice addition to combat in the game, but with it should also come the ability to cross the map to the other side to bypass enemies (something that would require speed and adequate defenses to survive).

We would need strategy settings such as retreat as a certain damage percentage, retreat immediately, bypass maximizing range to enemy ships, charge right through the enemies formation (this would give Wall some serious disadvantages while today it is the preferred formation). Incidentally, Ancient Greeks and Romans usually doubled their Wall formations to make it harder for enemies to pass through their formations. We would need a Double Wall formation added in.

Formations would also need to support large areas so you could attempt to screen a boundary. Of course screening boundaries in a two dimensional universe are so much easier than in a three dimensional universe. It is hard to mine three dimensional universes as well (hard to keep a uniform distribution in a volume when orbits cross planes).

A larger tactical map where ships do not start on the edges is a good idea. Ship speed would then matter more. Currently you only need tactical speed to match the range differential so that you can always return fire.

Currently in the game because of the way combat works (non incremental movement) speed does not matter much in combat, unless you are trying to board or ram or avoid one of those(use max range).
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Dream Land:

Now if SE 5 had an inertia based movement system that would be neat. I have not encountered a good strategy game that uses that yet. Think of the things you could do with formations attacking as they pass and turn around and go at it again.
Weapons with fast reload rates could fire several times during a pass while the slower weapons would only get one shot. Ships that have their engines damaged would not be able to turn around as fast and match speed with their formation and would break off. Order parameter would set whether the formation slows down or leaves the slower ships behind. Maybe Tractor beams could be used to tow lagging ships to keep up with the formation's course changes. Of course every ship would need a movement vector showing its speed and course. Once you do this you probably need to go to a point system instead of squares.