I'm not sure if this has been suggested before (I did a search, but I wasn't really sure what to search for).
Correct me if I'm wrong, but I think the current system has a minimum population growth of 2% once you reach 1 billion. Above that amount and it never falls further, no matter how high the population gets - even if you have 500 billion (I know, that would take a ridiculously long time to reach, but still).
How about an overall maximum population on worlds based on their size? Then you could have the population growth decline further past the 1 billion mark based on that size, eventually reaching 0 where the population growth would level out with as many births as there are deaths.
The reason I suggest this is, as it stands, the only need to ever colonize other worlds is to boost your population growth a bit - but once you reach a certain population level the minor boosts in growth you get from having colonies doesn't make that much difference since you're always getting the 2% on your homeworld. It's still useful to have worlds for minerals and bases to maintain your ships, but the entire financial, research, and construction capacity of your empire can be housed on Earth - even if you have billions upon billions of people. By limiting worlds to, say, 400,000 x diameter (or some similar number, whatever is deemed appropriate for gameplay - 400,000 would give earth a 5.1 billion limit) it would make colonizing worlds more important in the long run, since to support your 500 billion people you would need a large empire of worlds instead of just a ridiculously productive Earth. And even before you reached the limit, you would want to move the people off Earth since the growth would be constantly declining as the population got higher.
It could just be divided out and have the growth rate decline linearly until the limit is reached. So for example a 5 billion limit at 1 billion growth would be 2%. Add a billion and you're now about 20% closer to the limit, so growth declines 20% to 1.6%. At 4 billion you would only get 0.4% growth, eventually declining to 0 as you approach the limit. Over the limit would make growth negative, so you couldn't just farm colonists on other worlds and send them to Earth.
Thoughts?