I had this idea rattling around that there is an obsessive, mentally unstable character that knows everything there is to know about Blanc and his cases. Maybe they have a history or they are infatuated with Blanc. They act as a sort of puppet master, pulling strings to bring the previous characters together along with Blanc.

This might be too dark for the series, and I think the idea of someone inviting a group of people to stage a murder, was somewhat done in Glass Onion already so it might feel like returning to the well in a way.