The Rhino UI is written in Eto, a wrapper for WPF on Windows and Cocoa on MacOs. Of course you can also directly write WinForms or WPF. If you don’t need to support MacOS and therefore not need Eto, I would develop WPF in a dedicated DLL and just call the views in Rhino. You could also use any other UI framework (E.g. Avalonia), you just need to ship it with your plugin then. Any of the larger frameworks should allow you to create dynamic inputs boxes.