Yes exactly, a kind of shadowy mentor. Like Ahsoka in Star Wars Rebels S1.
Hell we could work a new cast into this.
Yes, Dian as the mc. Shes gonna need like someone next to her. Holly Blue? i think shed be so hilariously bad at being mom it'd be great.
Maybe Jasper, who sees what kind of shit Pink Diamond has descended into. Some of the Off Colors could fir. Rhodonite would shine here IMO, itd be a good opportunity to actually grow some balls cause shes a bundle of nerves (consider that by gem type, shes supposed to be stronger that Garnet)
idk who else
The plot would be split into following Steven locking down, capturing the remaining CGs (a 2 parter episode for each capture, one going after Amethyst, later Bismuth another after Garnet, and then Peridot. Lapis is the final boss of his arc. Each episode highlights how much he has fallen, as a the antithesis of what each of these gems taught him), while BD tries to stall him as quietly as possible. Meanwhile Dian and her team being sort of an undercover team using covert method to slowly build up a rebellion. All the while BD prepares for the innebitable larger scale conflict to come