Paper, by FiftyThree, just ran away from the IDSA awards with a gold medal. The UI is organically smooth and delightful to use. This is apparent in so many ways: pinching to open and close books, and incorporating a digital loupe over your drawing instead of a conventional zoom is so ingenious, it's remarkable it never existed in the first place. I love using Paper as an idea bank, but as a digital sketchbook, I'd like to see an idea sketched out in Paper go from concept to a "finished" digital drawing.
One of the reasons why I would like to see the jump from a beautifully designed app based on cataloging ideas and quick sketches, to a full fledged drawing program is because drawing with Paper is so smooth and expressive. One of the key elements of Paper is that the design team embraces a digital line, instead of mimicking an organic one. This, like the integration of the loupe as a zoom tool, is a no-brainer. Another app I use a lot for digital drawing is Procreate. The makers behind Procreate do an excellent job of designing a system that mimics "real paint". With your finger, or a pen, you can push paint all around a digital canvas. But it takes time. The makers behind Paper have it right with creating a new digital line, suitable for the space we spend more and more time in as we begin to gravitate away from "real paper" in our daily lives.
Below are some quick sketches done using Paper.