I mean, the season finale has awkwardly stretched into the third cour, but we're still four virtues shy of a set as of last chapter, so there's no way that's the end. So much merchandising to be done still…
ALV is Chiro's story, but a lot of what the framing device emphasizes is how little she values herself. Ending Shining Virtue Angelic Heart with Chiro's self-actualization would feel personally feel a bit dishonest in that sense. It'd also really deny her the chance to change that, to be a part of the main cast for real.
A sequel would probably pick up either a few episodes later or, more likely, post-series entirely, and I'd really have to consider what to do with the framing device in either case, since it wouldn't be able to carry the same meanings in a sequel.
I'd honestly be interested in a Gula/Temperance prequel with her whole arc to joining the Saints, though for a sequel I feel like Ida could be an interesting POV character, given she's been kinda stuck Just Being There while most of the bigger drama happens with everyone around her. Though yeah, Lupin/Avaritia would also be a fun and compelling focus too. I can't speak too much to a sequel though, not being caught up yet, and I should probably stop reading the thread notifs until I do ^^;
I probably wouldn't write an entirely Ida-centric spinoff. The whole mess of magical girls, adolescence and gender are a lot of what drives my passion for the project and Ida and Inessa are the furthest from that of any of the cast. Which is, perhaps, why they're the main two of the the actual show.
That said, if there's something. I'm unhappy with in the work as it stands, it's that I don't make as good a use of Ida as I do any of the others. Part of that is that she's the most distant from Chiro, but I would definitely want a sequel to do a bit better by her.