I love the idea of letters being hand-written. In fact, having just done the requisite minimum amount of writing with my fountain pens to keep them working[1], I wondered about building an app which can record you via the camera hand-writing a message, remove your hand and perhaps the pen from the video, speed-up and create a time-lapse video which can be sent to someone. Add a background from wherever you're holidaying (our desks for now, haha) and it's a modern-day postcard. I don't have the specific skills to build it but would love to show the time-investment in the process when sending someone a message. I'm sure that time-lapse video apps already exist, just not with the ability to auto-remove objects within the video.

[1] The two Lamy Safaris I have dry out if left pointing upwards after a couple of days. They're ridiculously wet and concentrated if left nib-down. Somewhat hit and miss if they're left level. The Waterman and Pelikan are both unaffected by their direction of repose. The cheap and now very old Parker doesn't seem to suffer either.