Though there are proposals for terraforming planetary bodies to suit humans, it is far more reasonable to create artificial habitats, that orbit around the sun, and / or the planets.
The construction of such large habitats will not be economical if based on Earth. Such construction will have to be accomplished in space, preferably by automated processes.
A fabrication unit, capable of operating from solar power, builds tools that build the tools that build the habitat shell, as well as new fabrication units, to travel to other raw material sources. Thus a geometric progression of habitat manufacture can proceed.
Utilizing near Earth asteroids would simplify and minimize launch costs for the initial fabricators. As parallel construction processes combine results, an ever growing industrial base would incrementally and geometrically expand. Once the empty shells are complete, then human crews would be dispatched to customize them for their respective occupants.
The benefits of colonizing outside of the gravity well of a planetary body are:
- Full time access to the nearby fusion reactor - the sun
- Minimal expenditure of energy / fuel to ship cargo and passengers to other colonies
- Fully customized microgravity environment
- May adjust orbit / task / position
For visualization purposes, imagine a twin cylinder O'Neill style habitat. This habitat could be configured to hold various climates, wildlife habitats, agricultural functions, and human habitat. As a self contained ecology, powered by the sun, it would be independent, and require only minimal resupply. (It may even have a raw material resource in tow)
Such cylindrical colonies might be attached to or travel with a large resource body, such as an asteroid or comet. By keeping in the shadow of the body, the colony would be shielded from direct exposure to solar storms and high energy particles. Long trains of cylindrical colonies might line up in the shadow, not unlike bacilli.
Traveling across the solar system could be simple, by shuttling to swarms of colonies, whose orbits intersect two or more planetary bodies. A traveler would 'hitch a ride' from colony to colony, slowly but safely transitioning to each new location.
The benefit of geometric expansion of orbiting space habitats, is that at some point, the rate of colony production will outpace the geometric growth of human population. As humanity and other earth life expand to fill the niches available, there will no longer be a competition for terrestrial resources. And if ever a catastrophe strikes Earth, it would not spell the end of life in the Solar system. And the colonies would be able to repopulate or restore the damaged planet, as needed.
Assuming geometric expansion, the solar system should provide enough resources and space for at least 2500 more years. By then, our descendants may have new avenues to expand along.