Lot of interesting and good ideas. I particularly liked the idea about different types of invading troops, like having armored divisions, shock troops, etc.
There seems to be a divide between those who don't want to make invasion too detailed or multiple turn, and those who do. Someone suggestion having the option in the start menu for detailed invasions, probably a good idea.
There doesn't need to be much change if the design sticks with 1 turn invasions. Spruce it up a bit, maybe have a few more options. But if one of the aims is to keep it simple, the range of options is limited.
If drawn out invasions become an option, how drawn out should they be? Little mini-games? Hex by hex battles? Like a mini game of risk but able to call town artillery fire from the heavens? Should an invasion, each invasion, become a significant event unto itself? Maybe be able to design land units in the ship editor? At-Ats? That's probably going too far.
What I picture is (as an option) an invasion taking multiple turns, but instead of seeing the fighting, you get the info in a way that is similar to the diplomacy screens... a general pops up telling you what is happening, and gives you some options, like 'send more shock troops' or 'commence bombardment on their defenses.'
General Mustache: "Commander/Your Grace/Speaker, the invasion is going as planned. We should be taking the capital soon."
You: "Excellent General. Report back when the capital is ours."
[Relatively equal forces/low random roll:]
General: "Your Excellency, the defenders of Planet Choo Choo have ion cannons in the mountains that we cannot reach. What are your orders?"
1) Bombard them from space (available if you have a fleet in orbit with capable weapons)
2) Order in the heavy armored divisions (available if.... you get the idea)
3) Attack them with our biological systems
4) etc. etc.
Each option would have a different likelihood of success based on the a) random condition, type of world, c) type of defensive forces vs. your type of forces, and d) on your technology vs. their technology.
[Very bad random roll, underwhelming force:]
General: "Uh, sir, its not going very well here. We may need some additional support. Soon."
You: "Grow a pair, General."
If we have Admirals added to the game, maybe we can have Generals added too.
If we have Admirals added, dear god I hope we can edit their names and ranks.