Auto line (ruler) and shape tool
When you need to draw a perfect line, rectangle, circle etc.
Saved Color Palettes
I would love to save color palettes and/or have favorite or starred colors. I love to work with some basic colors sometimes so I enjoy having primary and secondary colors in my tools at all times but when I’m switching projects I have to scroll and add those colors manually so I would love to either search by name for colors or simply have favorited colors to save that time for actually creating.
Scale / resize strokes
When selecting objects with the Lasso Knife, be able to resize them.
I'd like to be able to duplicate a drawing. This would let me try a few different variations on my original drawing without worrying about losing it. I could also use the original drawing as a template - e.g. a comic strip
A key advantage of digital over paper is ⌘F. If I could Find In Document, Find In All Documents, and _especially_ find handwritten notes through Spotlight integration, I’d subscribe in a hot minute.
Vertical Scrolling Documents (Portrait Mode)
An option for documents to scroll infinite vertically instead of just horizontal.
One of the things I love about Flow is its simplicity, so I don't mean to advocate for adding a ton of new brushes. That said, I think it’s fairly common for sketchbook users to carry around a tiny water color palette and brush and so would like to suggest this as a particular kind of brush which might be reasonable to have in this kind of app. (I noticed in the help documents a water brush was mentioned, not sure if this was an accident or a peek of things to come.)