My thoughts: Current system seems ok with some adjustments.
Planet should have base food production, which supports a base population, for example we won't live on mars till we can produce food on mars.
Planet Type should influence how much food can be grown on a planet. A desert planet will likely have much less food than a lush planet. i.e. fewer people live in a desert than in a lush environment. So a desert planet will have a lower base population than a lush planet.
Planet size could play into this base factor as well. A small lush planet will likely produce much less food than a larger one.
- Base Farms increase food production
- Planet resources used as currently do (build better farms).
One thing with "resources" being required for different improvements, it would be nice to see variance in them. For example you can have run a factory with Coal (resource), or nuclear power. Two different resources for improvements. Though clearly they have different effects on said production.
So in the tech tree, easier(earlier) techs require none or easy to get(abundant) resources. while later techs require more rare resources. This would prevent the problem Taslios mentions in not being able to build an improvement because no resources. Everyone should be able to build easy/low tech improvements with easy to get resources, while the later techs require harder (rarer) to get resources.
One thing I don't like about the the current resource implementation is that mid to late game most resources become worthless. This may be a trade issue, but sitting on 1000+ of a resource is like why do I even bother? Only the few space bound resources (i.e. antimatter, etc) seem to be of any value and even then Thulium, compared to others seem much less valuable.
So using ideas above, coal plant or solar power. Two different types of of resources. If I only have coal I will use that, which could be bad for planet (moral, or other impact), while solar power would be better.