Per my comment above I have a prototype of this feature up at https://github.com/chiaracoetzee/fluxer-subprofiles/tree/features/subprofiles based on @haileyscommit's design and it's working well on our home server. It's web/desktop client only not mobile right now. Would need a lot more prep before submitting a proper PR. Supports both GUI subprofile selection and PluralKit-style prefixes, and can import a system from a PluralKit JSON export dump. Tested with a large imported system with 60+ members. Some screenshots below for reference.
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Also, I think the vocabulary is a bit inconsistent. In one place we're talking abut "subprofiles", in another it's "personas", and in another it's "subprofiles & personas". I think it should just be "subprofiles", it's the most neutral word that corresponds to the broadest application of this feature imo.
I'd argue that "personas" would be better to settle on because it's a more distinct name, whereas "subprofiles" could be misinterpreted as server-specific profiles.
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Comment by @chiaracoetzee
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Comment by @haileyscommit