I would love to see cloud features like those in the Nasa images from orbit, with moving cloud/weather formations on the gas giants. And should we ever get atmospheric flight, things like the first image. Would be incredible to fly through that.
Speaking of which, it would also be nice to see moving cloud/weather on the rocky planets with atmospheres, including the Earth-like planets. I realize these things are often slow moving when seen from on high, but nevertheless clouds move and change over time, so it would be nice to see that in game.