As an example I'de like to mention the calendar screen template where a calendar has been created with 2 galleries.
One for days of week and one for the monthly days and additonally there is a month selector.
If it was possible to select these controls and save then as a custom control (calendar control) then it would speed up development for future projects.
Sure would be nice!
For the meantime, it's possible to use the copy-paste strategy... But it would be nice to create our own controls and even have the possibility to change it on a later date in one place to change it everywhere.
A good example would be for handling files. Right now, I use buttons, a gallery and an "Attach File" control for it. In a near (Hopefully) future, PowerApps would provide a better solution that I could implement without undoing most of my existing apps.