we've opted for cool over realistic since, in all likelyhood, most planets are either just big rocks, big ice balls, or balls of gas. So having worlds made out of diamonds and rubies and gold and iron and other weird things allows players to constantly discover new things to explore.
That approach sounds fine to me. And in reality diamond worlds is possible, tweaking it a little and adding some weirdness, and finding exotic phenomena/minerals etc.. will make it exciting. Tweaking of some realistic worlds could work very well keeping the "realistic feel" if you look at a swamp world which is realistically possible according to scientists, instead of the green it reflects, create a world that reflects red in colour, but make it look realistic from the outside as it would possibly look if real (it could be due to volcanic flows)
if you look at earth i love seeing the continents, the blue ocean and the clouds moving around the earth exactly as is, it could be hurricanes/super hurricanes/super lightning,super volcano's on other exotic worlds, seeing lightning randomly flash on the world would be super cool. or lighting shooting off it towards space.
Then just a crazy example: reverse hurricanes when your lander flies into it, it will shoot you off the planet back into space. if it can be made to look real.
( just as an example of how the creative thought process should explore in exotic but possibly realistic or realistic looking ways)
Then worlds like:
Super earth's, swamp worlds, magnetars. pulsars (only possible to fly to with a special lander/upgrade)
like Titan and Europa.
How about a strange world where you see something akin to a black hole hovering/moving around on it's surface, so instead of a hurricane you see this black dot with fierce gas/clouds/matter hurtling around it orbiting on the surface of the planet as you would see hurricanes on the earth from satellite images. That is something that would seed curiosity.
imagination can run quite wild but try make it look realistic, even if not really possible. Seeing the earth as is from the outside moving and all the weather being active looks awesome already.
So Realistic would be Planets with crazy tectonics and weather, fire worlds/ ice worlds to start off with, then tweak others from there.
I feel it important to encounter worlds/phenomena out there that is based on what is realistically possible and what science know about what is possible and out there, after all, those planets are crazy and exotic by nature (yes tweak it here and there but make it look real) It can be tweaked but making it look real and possible will create a winning formula and stoke excitement beyond anything remotely comparable.
Leave the "gems" only for landers that are equipped, upgraded to be able to handle the conditions. (which i'm sure is already how you've done it in SCO)