Up to your imagination!
Personally I will probably use them to integrate 3rd party
tools or create small helpers in my day to day work that are
currently small bash/bat scripts. Using D for scripting this
way would be very nice imho.
Of course the editor itself can (and does) make use of such
widgets to show misc. info about you project state.
Could you put the code on github or somewhere so I can have play
with it? I don't care what state the code is in or if stuff fully
works or not - just want to have a go. I'm writing an intellij
plugin for D https://github.com/kingsleyh/DLanguage and curious
to see other approaches to editing D code.