I’m also missing this feature, coming from a Laravel world.
I have a “main” master page with the top/header navigation bar, and for logged in users I present a dashboard which is basically the main master page with a sidebar as well.
It would be awesome to create a “dashboard” master page (containing the sidebar) based on the “main” master page. The alternative is attaching a “sidebar” partial to every dashboard page, but it doesn’t look as nice from a development point-of-view.