Handwritten notes
Gnome (and Linux in general), is missing a good handwritten notes application for digital pen input.
We have the ancient Xournal and the much more recent Write, which is awesome but is not open source and doesn't seem to be being developed anymore.
A modern GTK responsive app would be awesome to have, especially with the rise of tablets, 2in1 devices, and even Linux-based mobile phones. It would turn using Linux on those devices into a more complete experience, helping it becoming a more feasible alternative to other OSes.
Ideally, it should use a standard vector format like SVG and have text OCR capabilities to enable text conversion, search, etc.