Hi,
I'm new to the game and there were a few things that struck me as i played the game. I don't know if it has been suggested before, but i didn't find anything with a quick search.
The first idea is that it might be cool if planets were a bit more diversified. By that i mean that they could have different atmosphere, for example, oxygen, CO2, hydrogen and so on, along with the type of planet they are, like radioactive, aquatic and so on. I guess a game that has something like that is Space Empire V. I think it would be more fun to explore.
The second idea is that if races had a favored planet type/atmosphere, it would also diversify things a bit more. Different planets will have different values for each race. Some race would have penalties on a planet, while another race would have a bonus.
The third idea has to do with population. In a lot of other games of this type, you had the choice to exterminate or enslave the population of planets you invaded. The difference is that if you captured a planet, the population of the planet would be of the race it belonged to.
Maybe it could be interesting if instead of just a population number that you have in galciv2, you had the race also. Then you could interact with them, like sending them off to other planets of your choice and i'm sure more ideas could be found.
One advantage was that if previous suggestions were implemented, you could use them to colonize planet types that fit them, negating penalties your own race would have. There would have to be restrictions so it doesn't get overpowering, perhaps having more races enslaved = harder to control by have a big morale penalty or some other restrictions.
There is mention of enslaving races from the evil races, perhaps this could be the way it is represented in the game.