Saturn

Terraforming of Saturn seems impossible, but if it should become a habitable planet for humans, we would have to put a giant artificial surface with low altitudes, filling them with water oceans to support life. It could be built around the upper atmosphere and the top atmosphere can be altered to be nitrogen/oxygen rich and could be warmed so it can support life. The artificial surface could have huge super-continents and oceans. The amount of raw power and resources it would take is unimaginably high, but it's plausible that the process may be finished in a very distant future.

From Wikipedia:Edit

Theoretically, any gas giant could be enclosed in a solid shell; at a certain radius the surface gravity would be terrestrial, and energy could be provided by tapping the thermal energy of the planet.^[22]

A more realistic plan, which, at least theoretically, would be possible in a more close future, is to react carbon with the abundant hydrogen, to form methane; a powerful greenhouse gas. If sufficient amounts of methane are formed, and the temperature is raised to −33.34 °C, ammonia-ice clouds would evaporate, contributing to the global warming. However, methane is mostly carbon by mass, thus the only way to create sufficient amount of methane, is to either bring an enormous supply of carbon, or to create carbon by nuclear fusion. Both of which, are beyond our current technology.

Oxygen would also have to be brought/created in great Amounts to react with hydrogen in the atmosphere

Because of the sheer size of the planet, constructing artificial continents is very impractical. Floating bases would be constructed for humans to live there. That presents another problem; the Saturnian atmosphere is 96% hydrogen, the lightest gas, and the rest being helium; the second lightest gas, which makes it problematic to hold the base in the air. There are three ways to go around the problem:

-Use a hot air balloon instead.

-Use a solid shell, that would contain less dense air.

-Building an enomous balloon, filled with pure hydrogen, to compensate for helium in the atmosphere.