This Master of Orion 1 style ground combat is one thing that is lacking in this game and needs to be improved. I know a patch probably wont be enough so this is something more that maybe would be included in an expansion or sequel but heres my idea.
Why not have military units? Something that could be or even doesn't have to be designed by the player. You could go one way and have units that come as you research. Like, for example, you start with the basic soldier but can research power armour, tanks, mechs, aerospace fighters, etc. This was is simular to something like Master of Orion 2 where you build these units and they can guard a planet, then you load them onto transports and send them to attack the enemy. You then take the planet and the amount of occupation forces determines if you can hold the occupation that could have chances of rebellions.
Then their is another way simular to Galciv2 space combat. The technology you research for ships can be applied to ground units. Small would be infantry, medium would be tanks, large would be mechs, and huge would be something else. Infantry can only use beam and ammo weapons or small weapons in general while tanks and larger can use missiles, etc. You can put more on the larger unit and then when you invade combat works simular to space combat except the defender should be receiving certain bonuses. You could even have different kinds of units, like a air unit that is more expensive and is good against almost all terrain but can't occupy, etc.
You can even add in other aspects. Like have it so that infantry get bonuses for urban, mountain, and forest combat. While tanks get bonses for plains, desert, and simular warfare. Also you can have where infantry get bonuses for occupation where other units do not so that all ground units have to be used to be successful.
Though certain other things would have to be changed as well. Like, for example, you would have to add the fact that their would be aliens on that planet with even possibly different bonses which would probably be hard to implement. Which is, why I said, it would be better for a expansion or sequel.
Its my opinion that ground combat in this game is lacking. Others would agree or disagree but I personally like it to be more complicated than you have X technology and X solderiing here are your bonses you use X civilians to invade, the planet is yours without trouble.
This way it adds a whole new set of challenges to the game and would keep it interesting if, for example, you have a large amount of conquered population. Also i'm sure that this would add the chance for a LOT of flavor for evil or good races. Like citizens conquered by good races of evil alignment might switch to liking the empire more quickly. While evil races can use the good races as slaves, etc.
Like I said, this is definately NOT something i expect them to add. Its more of a sequel or major expansion kind of idea. I got these ideas from Master of Orion (first idea) and Space Empires 4 (2nd idea). Of course if they approached this they could probably come up with even better ideas but this was just a quick thing I decided to write. What are your opinions on this?