Couple of suggestions:
With SpaceX lowering the cost of space transportation, many new options become viable. I like the idea of mass drivers on different solar system objects sending stuff to other places. We can even move large objects over thousands of years such as Mercury with rail guns. Given a million+ firings of rail gun we can push Mercury further away from the Sun. Eventually we could make Mercury a moon of Venus.
Venus is hot because it has a dense atmosphere. Venus atmosphere is 160 miles deep, while Earth is 62 miles deep. If Earth had an atmosphere of 160 miles deep, Earth would be similarly hot as Venus. This suggest we can cool Venus by removing atmosphere. Analogy: the higher you go up, the colder it gets.
Lets take hydrogen from Jupiter, ship it to Venus and convert Venus carbon dioxide to water. Water will reduce the amount of atmosphere, thus cooling at surface.
Another option is with Mercury as a Moon of Venus, scoop up carbon dioxide and send it to Mars using the railguns.
We can increase the mass of Mars by sending mass to it from all over the solar system. We can even send large objects like moons to Mars over the long run. With increased mass, Mars can hold a thicker atmosphere. With a thick atmosphere, Mars will get warm.
3D mega cities. We can connect sky scrapers with neighboring buildings with bridges. Bridges can carry traffic. This would alleviate traffic and elevator issues.