This is my second attempt to tackle the planet-side of GalCiv. For my first attempt, duly settled in my opinion, see https://forums.stardock.com/456574/page/1/#3485406
To be very clear right from the start, I will not be proposing a game within the game for ground combat. What I'm about to propose relates essentially to game flavour.
And yes, it does involve some increase in micromanagement.
Instead of a square grid, planets' surfaces could be divided in sectors with irregular boundaries, like real countries and provinces, or if you prefer like a board of Risk. This doesn't change the game mechanics, but it would make the planets look more like planets, in my opinion.
I know a square grid is easier to program, but what I'm proposing is not that much harder. All that is required is a certain number of ready made quadrants, each with a set of pre-designed sectors, that could be combined like puzzle pieces, or Carcassone pieces for those familiar with the board game. 10 or 20 such quadrants, combined in a 3x2 array, could give rise to thousands of different planet maps. Note, just to be clear, that this paragraph is important only for the gamedev side of things, players would only see the final maps.
Accepting suggestion 1, I propose dividing the ground improvements in two categories: sector improvements and local improvements.
Sector improvements would occupy an entire sector. These would include industrial sectors, farming sectors, military sectors and urban sectors. Each of these improvements would change the sector texture, further individualizing each planet.
Local improvements would be built within already improved sectors, never in unused sectors, and would enhance or modify the sector improvement in which they were built. They would appear as smaller icons overimposed on the sector texture. A sector could have between 1-5 local improvements.
I see the starport as a sector improvement. The way I see it, it's just not an airport to space, it's a huge industrial compoud dedicated to spaceship building and launching.
Accepting suggestions 1 and 2, I propose changing the way the game calculates and displays planetary invasions. This is the only suggestion that actually would change the game mechanics.
What I propose is that each sector and local improvement would have a certain military contribution, expressed in terms of number of soldiers. The military strength of a given sector would be the sum of its base military value with those of all its local improvements.
When invading, the attacker would be informed only of the planet's global military strength. Of course, looking at the map a player would be able to estimate the strength of individual sectors.
The only decision the attacker would make would be in which sector to launch the initial invasion. After that, combat resolution would be automatic. In my opinion, invasions would be more realistic if combat resolution lasted several turns. A link to the combat situation in a disputed planet would appear in the report at turn beginning.
The method I propose for combat resolution is somewhat complex, but it could provide interesting results, I think. In the initial invasion, the whole invading force would battle only the invaded sectors' forces. After the forseable victory of the invaders on that first sector, combats would be set between neighbouring sectors. The forces in each sector would be divided in as many parts as the number of neighbouring enemy sectors and all combats would be resolved separately. If a sector is left without any defenders, half of the attacking forces move into that sector, the other half remaining in the sector where they attacked from (to avoid leaving sectors unguarded by victors). An invasion is over when one of the sides has no more troops to fight.
Note that the players would not see any of this, as combat resolution would be automatic. Each turn, the players would be informed only of casualties and sectors gained or lost.
I haven't tested this combat mechanics, but I'm fairly confident it would require the invaders to outnumber the defenders, which I find to be reasonably realistic. Choosing a good place for the initial invasion would also be critical, which I find interesting. Bombarment from space could even the odds, by destroying local improvements (but not sector improvements) and weakening the sector defences.
An additional option would be to allow reinforcements on either side as long as combat is active. It could be interesting, and not that difficult to implement once the combat mechanics are duly coded.
Finally, something would have to be done about third-party interference in planetary invasion. Option 1: during combat, planet was off limits to other civs. Option 2: during combat, planet is off limits to all civs that are not allied to one of the fighting sides, allies working as reinforcements. Option 3: during combat, invasion by a third civ not allied to any of the fighting sides amounts to declaring war to both sides.