The concept of the gems and rock people can be retrofitted into a badass scifi universe if you put your mind into it. Thing is, it stops being Steven Universe and becomes something else entirely. SU revolves around Steven crying and lesbians talking about their feelings. Take that out and the series... Changes. For the better, but you might as well give it a different name.

As pointed out in another thread, Steven himself can be salvaged by having him be a normal boy that grows into a true badass like Enzo from Reb00t but... That's not Steven anymore. His whole character is being an effeminate fat kid. Hell, the whole concept of a rock species harvesting planets can serve as a kickass backstory with vengeful species and homeworld wrestling with the ghosts of it's sadistic past but again... That's not Steven Universe.

Christ. If I did it:
>The Chrystal Gems are now the MetalMen.
>Pearl is Bronze. A hardened warrior with disdain towards the lesser races, but his respect for his mentor makes him beholden to Steven.
>Amathyst is Copper. A brute who has more in common with the savages found in feral and feudal worlds. Defends Steven as he's honor bound towards his chief even after death.
>Garnet is Iron. A soldier with a real killer instinct but regrets the war and what they did.
>Steven has a healthy circle of friends and lives a happy life unlike what we... Got in the series.
>Rose is Sterling Silver, and he made Steven as an attempt to atone for his role in destroying worlds for his Empire.
>Girl Greg has more in common with Hilda's Mum with a relatively normal job.
>Beach City is a metropolis and the side cast is massive: Steven has an entire life outside of the Rock Men and has to juggle his duties towards his fellow aliens and his human side. Sterling wanted Steven to be happy and would prefer the latter than the former.
>Many alien species are encountered, as are extradimensional entities and robots.