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.)