About the advanced panels, I was thinking to use the Go language with the Qt binding for it. It is cross platform and very well done!! Also in this way I will learn Go (something that I want to do ) a little bit.
To exchange data between Sublime Text and the advanced panels, I think I will use stdio transport defining for each request/response a Content-Length in order to get the message correctly and asynchronously.
This is similar to the test that I have done with the Tkinter Python GUI module, but in this way the user doesn't need to install python 3 on its own because Go code will be compiled for each OS! Also this framework is much more powerful compared to the Tkinter module