I don't think I'll ever write a sequel or anything, but while I was working on STAR GIRL, I played around in the universe's sandbox a bit. I have a lot of thoughts on all the minor characters.
There's a future where Charon becomes a vigilante magical girl in my brain, and there's one where he stays inactive. Either way, that story would be centered on his younger siblings, Sunny and Aurora. (Much like STAR GIRL was centered on Charon, Stella's younger sibling.) I think all of them are pretty messed up for a long time post-comic, but Sunny and Aurora come into their own when they're at college. I think they'd end up protesting the conscription of kids as magical girls, and the government's apathy toward monsters... The antagonist would definitely be more clear-cut than in STAR GIRL.
In the past, I think Fire Girl and Garden were pretty good friends. They worked together often, and got along well even though it seemed like they were bickering all the time. When Garden died, Fire Girl quit being a magical girl, and she never got the government retirement package because she didn't technically retire. So she is not well-off at all, money-wise. I think, during STAR GIRL, she has a girlfriend, though. She's 20, and they're living together and making things work as best they can.
During the comic, V and Cirrus cross paths behind the scenes. Both are 14, and Cirrus has already turned out to be a very jaded person. She's by far the least personable magical girl, and she never voluntarily joins in on any publicity stunts. If somehow persuaded to join, she is so harsh to fans/interviewers/etc. that the agency always ends up sending her home a few minutes into the event. V is the opposite, a people-pleaser, though not because he enjoys the fame. He gets very anxious under the spotlight. Cirrus thinks he's too nice and complacent, but she still can't resist the urge to look after him.