If Garnet is Ruby and Sapphire's child, what about Sardonyx and Sugilite and (more importantly) Opal?
Hmm. Maybe they could be experiments by the government/the diamonds, taking the role the Homonculi filled in FMA? Plus they can be dark reflections of traits the crystal gems possess or something.
Hm. Other rebels trained specifically by Garnet and Pearl and Amethyst, maybe? Rebels who pull from the specific alchemy styles that those three have in specific ways? (Which opens up the avenue for Alexandrite to be an unofficial Dragon's Breath Alchemist.) There's still room for most/all of the CGs to have been killed during the rebellion while having fusions be the few remaining. Maybe they went off to try and do their own thing, and Steven&Co only run into them in certain areas, or if the CGs have to call them for help-- I could see Sugilite trying to incentivize miners to riot for instance.
Glad you think so. and what about Bismuth? Maybe she can be a retired state alchemist (or an ally of Rose's from Ishval) who works as a blacksmith and fills a role similar to ["a housewife", i suck at names] except less of a direct ally. Or she can even be more akin to Scar, not willing to give up the fight against the Diamonds after what they did during the war, even though it's ruining her own life.