this sounds great!
At the moment i made the buttons and statusbars with images. Taking into account the other cool features in your roadmap, i would put the Rectangle -stuff to the features list with priority 2.
Regarding the plugin system: i.e. for local builds on desktop i can add the qml-files as resources and compile them into the binary file. I think this will not be possible with a plugin system, or am I wrong? Is there something planned to protect the files from unauthorised access?