Apple is said to be working on a new feature that will improve the management of external displays including allowing the use of an iPad as external display.
According to people familiar with the development of the next version of macOS Apple is working on a feature that will allow users to send any window of any app to an external display. The external display can be a traditional external display or the users iPad.
A new menu will be opened by hovering over the green maximise button on the top left of windows in any Mac app. The menu will allow making windows fullscreen in the existing manner or by moving them to an external display.
When using an iPad as an external display with an Apple pencil you will be able to draw on the display and use it like a Wacom tablet.
The internal codename that Apple is using to refer to the upcoming feature is Sidecar. It is expected to be announced with macOS 10.15 at WWDC in June.