Use phone as a drawing tablet (aka pen tablet, digitalizer tablet)
Currently we can send mouse input from a phone to a PC. With minimal changes, this could be used as a way to draw with your finger/stylus on your phone into drawing programs like Gimp. The changes needed are to allow absolute input positions (where each corner of your phone/tablet maps to each corner of your desktop screen) and simulating that the left mouse button is pressed while you drag your finger/stylus on the phone/tablet screen.
When using drawing tablets (eg: Wacom ones) as an input device, programs like Gimp can obtain info about the pressure applied, the angle of the pen or the size of the tip, which they use to affect the drawing. Both Android and iOS provide APIs to obtain this same info when using a stylus on the device (and some even just using the finger). A way to further improve this feature would be to forward this info to the PC so we could simulate the input from a drawing tablet using your existing smartphone/tablet. [Note: this should be split into its own task once the initial, simpler version is done.]
Related ticket on bugzilla: https://bugs.kde.org/show_bug.cgi?id=445511