Here are my ideas on how Jupiter and Saturn should look like.
Jupiter: An open field of grass with rivers and some fresh, green trees.
Enemies: Fallen, Hive, Vex, Cabal
Reason: Jupiter is the largest planet in our solar system, that being said, with more land they'll have more soldiers or whatever.
Saturn: A grand metropolis filled with skyscrapers and lit up buildings. (Kinda like a L.A, New York City, Las Vegas mix.)
Enemies: Fallen, Vex, Cabal
Reason: Saturn's rich and valuable technology can help them grow stronger.
Before you get all real on me, note: look at Venus.
Comment your ideas and opinions on how Jupiter and Saturn should look like.
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I'm pretty sure even the traveler can't terra form a non terrestrial planet ergo a gas giant. The pressure on Jupiter is only slightly above the pressures required for fusion to occur. You can't terraform mass away without violating the laws of physics, the gravity itself would be crushing. If the traveler could some how do all this there would be no darkness because the traveler would could control the universe.