Actually, For GCIII I suggest using the Reach For The Stars system. Each race had different tolerance for planets' atmosphere, heat, and terrain type. So for one race this junkyard of a planet would be nearly worthless, but for another, it'll be a paradise! Imagine the possibilities: should I bother to colonize this dump or should I just colonize the next planet that my race could flourish on? Should I colonize it just to deny it from other races and let them grovel and beg me to give to them in exchange for everything I want?
There should even be some planets that will be uninhabitable at all for some races, but fully or even a bit inhabitable for others. That could be really great. And each race could even be able to gain extra stuff from certain types of terrain types and planets. Like, for example - an aquatic race could gain happiness and breeding bonus in water-worlds. What do you think?
Oh, and ofcourse, there MUST be orbital bombardment in GCIII. Simply a MUST. Let it take awhile to level a planet, so it will not unbalance the gameplay (unless you use some REALLY heavy-duty weapons and fleets, in which case you could level a planet in a turn or two).