I feel that only a percentage of a population should be allowed to fight for the defending planet. Different races could have different percentages.
Also, a personal wish for me would be to overhaul the actual invasion. For instance, at the actual invasion, the world screen should show with all the inhabitable squares. Depending on the size of the attacking force in relation to the defending force, you can select up to 4 squares to invade (more with proper research). Your troops would be split accordingly.
Each square has it's own advantage factor for the defending troops. If a valuable military production upgrade has been built on a square, the defenders would have a higher advantage factor because there would be highly trained soldiers and the best equipment guarding such buildings.
Empty squares would have the lowest advantage factor.
While a planet is being fought for, it can still be taxed and build things, only at a greatly reduced rate.
The longer a fight goes on, and the more territory the attackers gain, the more the planet's morale drops. Once it drops below a set threshold, the planet surrenders.
You could maybe have a combat management screen similar to MOO3 to make it easier to manage the invasion and/ or defence of multiple worlds (*gasp* yes I actually liked some elements of the game).
This is just the bare bones of my idea but it's what I would LOVE to see in Galciv2.
I know it's not ever going to happen though, as I'm imagining it would require a LOT of work on the dev's part to implement.