Decent diplomacy options. I really liked call to power II, but every time I encountered the diplomacy system I wanted to hunt down the programmers, wire a computer to their brain and force them to use said options for all their future communication with other people. Then I wanted to tear their stomach out and feed it to them.
I’ll second what Krisolov said - I’d love a space based strategy game set in a single solar system. Ooh - how about this:
The solar system (not neccesarily our one) has been colonised - the major planets have been occupied and industrialied and people are turning to the asteroid belt, both for living space and valuable materials. Mostly volatiles and delicate carbon structures, but the asteroids are also surprisingly rich in other areas.
Unfortunately it takes a while before an asteroid is self sufficient, so a lot of stuff has to be shipped in from the inner planets. So, that war that broke out was rather inconvenient. Lets just hope that we have enough stuff to last until they can set up trade again, some time in the next thousand years while they climb their way back up from that stone age they were bombed into. Oops, there goes fusion plant number 3… Say, doesn’t that other asteroid colony have a spare? I’m sure they wouldn’t mind if we ahh… borrow it. 
And so it begins - the asteroid colonies don’t have enough for them to survive on their own, so some are going to have to fall. Build up your resources, split them between your military, infrastructure and research. Can you survive long enough to become a self sustaining civilisation? (And then conquer the solar system, as all self respecting civilisations want to do.
).
Geeks (I mean that in a nice way - I’m one) everywhere would love you to bits if you included the two following features:
Light-speed transmissions. Have a turn be a month or so - nearby ships will get your orders instantly (turn-wise), others may take a turn or two.
Accurate fuel usage - finding optimum paths (the computer would do a lot of that for you) in gravity wells, fuel storage in ships, reaction mass, etc. No reactionless drives until you get impressively advanced, no faster than light travel.
If you could have the planets and asteroids move right under gravity as well that would be stunning.
Gravity simulators aren’t that hard to program, especially if you don’t need a high level of accuracy.
Your civilisation would start out fairly small - little more than a couple hundred people. Perhaps an individualising system for specialists - pilots, scientists, etc. Pilots could have preferred maneuvers, scientists could have areas of strength, etc.
Allowing travel out of the ecliptic would be nice, but isn’t really neccesary.
This also fits pretty well with your low graphics requirement. Space ships can be little more than blips on the screen, with not much more required for asteroids and planets can just be circles with little detail.