I've had an idea for a while about a group called the stonecutters; A group of human gem beast hunters. Corrupted gems cover Earth, and the Crystal Gems can't be everywhere, so the humans created specialists to fight them on their own. They keep up with the corrupted gems physically by embedding the shards of shattered gems into their bodies, increasing their physical abilities the more shards they collect, as well as allowing the use of different powers and gem weapons if you have enough of the same kind of gem. However the process also cause schizophrenia with symptoms growing more severe the more shards are added. The voices of course are not delusions but the subconscious whispers of what little of the gems minds remain in the shards.
If you were to introduce one as a villain, he would likely be the last one, out to finish what they started by using all the gem shards they had now that the rest of the Stonecutters are gone. He could be a scary villain, you could see the gems horror of having to face a skilled superhuman That's basically covered in gem corpse parts, who's attacks methodically aim for their gems, but also rambles on like a madmad, occasionally mentioning things only gems should know.
Then Steven could psychic link with him to figure out how he got this way. Then you have a big moments of each side coming to understand each other, he learns all gems aren't monsters, the gems realize he had to do what he did to protect people. Steven and the diamonds could even fix the corrupt gem shards and Steven could teach him to get along with the gems in his head, more or less curing him schizophrenia and making him an occasionally reoccurring cast member who kinda sorta resembles the real life minority of people with schizophrenia, but like a weird space fantasy version of it.
There, crazy gem fight, big emotional moment, everybody's friends now.
Steven Universe episode.