Sometimes I toy with the (very) long-term idea of robotically mining Ganymede into lots of 20m cubes and using mass drivers to push them to impact on Mars to increase its mass. You’d have to do it robotically because the radiation from Jupiter would be quickly fatal.
Using the mass from a very large number of asteroids, basically clearing the inner Solar System of debris, and processing out the valuable stuff, would be worthwhile.
The tailings could be processed into a mineral foam and piled up in an orbital spot between Earth and Venus, or perhaps better yet between Earth and Mars.
The desired shape would probably be like this magnetic toy:
https://i.ebayimg.com/images/g/UW0AAOSw5b1hrQba/s-l500.jpg
Water is about 1/4 the density of rocks. A volume of water about four times that of the Earth would then be introduced. The proposed axial core would help keep this water planet in a pretty good oblate spheroid shape.
Mineral foam “frisbees”, miles across, would float, as a sort of artificial pumice, and be dropped to the ocean surface, and of course the 78/22 atmospheric blend introduced, along with Earth species. The floating islands would provide habitat, and quite a lot of it.
