The Widget Builder just got a whole lot more flexible.
You now can embed your own widget control panel directly into the editor.
This means that you, as a developer, do not need to rely only on the standard input options -- you can instead build with the unlimited power of the web.
The way this works is by embedding a page (via an iframe) you host into the widget editor. Then, within the website, you also add your own code to display the widget output. Since you control both the output and the widget control panel, you can have the user use the widget editor and communicate in the background to the widget on the website.
As you can tell, this is an advanced option, but it will give developers and others the possibility of building any widget they can imagine with their own backend service.