Assume for the time being that it will forever remain beyond the scope of science to change Human Nature. AGI is also impossible, as is Nanotech, BioImmortality, and those things.
Douglas Adams mice finished their human experiment, giving to you, personally, the job of redesigning earth, and specially human society, according to your wildest utopian dreams, but you can't change the unchangeables above.
You can play with architecture, engineering, gender ratio, clothing, money, science grants, governments, feeding rituals, family constitution, the constitution itself, education, etc... Just don't forget if you slide something too far away from what our evolved brains were designed to accept, things may slide back, or instability and catastrophe may ensue.
Finally, if you are not the kind of utilitarian that assigns exactly the same amount of importance to your desires, and to that of others, I want you to create this Utopia for yourself, and your values, not everyone.
The point of this exercise is: The vast majority of folk not related to this community that I know, when asked about an ideal world, will not change human nature, or animal suffering, or things like that, they'll think about changing whatever the newspaper editors have been writing about last few weeks. I am wondering if there is symmetry here, and folks from this community here do not spend that much time thinking about those kinds of change which don't rely on transformative technologies. It is just an intuition pump, a gedankenexperiment if you will. Force your brain to face this counterfactual reality, and make the best world you can given those constraints. Maybe, if sufficiently many post here, the results might clarify something about CEV, or the sociology of LessWrongers...
1. Take apart the earth, use it to build a fleet of smaller habitats that make use of the materials to generate maximum sustainable habitat space.
2. Distribute these habitats into combinations of nature preserve biomes, agricultural systems and urban centers.
3. All habitats should be given sufficient technological infrastructure to power all their life support with some extra. All habitats should have internet capabilities and backup stores of all human knowledge.
4. Redistribute the population of earth into their preferred habitat with the kinds of people they would prefer to be around, pole them if needed.This includes isolating ideologies that cannot abide any other ideologies away from other habitats.
5. Dedicate a small portion of habitats to ferry services that take regular routes between other habitats. Arrange routes so that individuals with ideological incompatibility are separated from those they cannot tolerate by longer routes.
6. Sit back and let the chips fall where they may. Live in a habitat with people of like mind, eventually die but know that humanity has been given a head start to interstellar colonization and the biospheres of earth have been preserved as redundantly as possible without dismantling any worlds beyond earth.