It should mainly take place on Homeworld. Earth is visited occasionally and we should get Spinel to visit all the unique outer gem colonies throughout the season, introducing unique new locations, gem lore, and characters.
It would be pretty boring just having Spinel hang out at the palace doing nothing like Future implied so either have her casually exploring Homeworld and the galaxy while making new gem pals along the way or forcefully have her involved in important events by having her taking up a role under the new political system established by Yellow Zircon or Blue Zircon that requires her to handle issues (social, political, or otherwise) plaguing Homeworld and the outer colonies due to the Diamond's reign at an end. The galaxy needing a peacemaker Steven and her trying to fill that role would be interesting to see while also making allies and enemies along the way.
Bringing in potential regular cast members like Nephrite, Lars, the Off-Colors, and doubtfully, Jasper and Pink Pearl alongside the Diamonds to develop them all further would be great. Introducing entirely new gem types to become regular cast members would be good to.
First season should be slice of life and simply introduce various concepts, ideas, and problems that Spinel and others need to experience and resolve while hinting on bigger problems for later on. The Cluster being fixed by Yellow Diamond should be done here.
Second season would continue the trend and involve the mysteries of Homeworld, the origins of gemkind and the introduction of another technologically advanced metal
race that has to be handled with care while building up another gem conflict between the Homeworld government and some outer gem colonies who have chosen to reject the new system put in place by Steven, including Eyeball, Aquamarine, and the main antagonist, Emerald which would then also heavily involve Lars and the Off-Color.
Third season would resolve the gem conflict.