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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
For example, the plex web API can show you which endpoints to hit to do certain things.
In the end, your Idea wouldn't really work because Plugins are not integrated into the client itself. You would need to write a tool that does this from the outside but that already exists which means you don't have to reinvent the wheel to do that. Still, I would recommend you get your library into a state that works better with plex because all those programs expect you to follow that convention.