There are several features that can be improved, need some tweaks, or need a complete re-design to reach a professional level for full-time pixel artists. These recommendations are thanks to cyangmou + some old notes/to-do items.

• Pencil tool: Shift key to start drawing a line from the last point with preview. Dev note: Shift key should start a Line tool ToolLoop, then releasing Shift cancels the loop or releasing mouse button finishes the loop successfully. (Related, fix bug Reset last drawn point after undo for draw line behavior #1005)
• Pencil tool: Ctrl+Shift (or other combination)
• New color picker (issues) with RGB/HSB at the same time.
• Contour tool: Shift key should start a straight line and releasing Shift should continue the regular freehand mode.
• New brush manager #3219
• Fill selection content changing FG color or other recolor technique. There are plans to recolor an RGB sprite as an Indexed image (all layers/all frames) which might be related to this new feature.
• Alternative mode to create planned rectangle/ellipse shapes before the final fill/stroke. (Ref 1, Ref 2, Ref 3)
• Fix rotated squared brushes. We need a new specific algorithm to create these brushes.
• Fix bug Rotated line brush pencil is thicker than expected #245 and Line brush can leave gaps #728, continuous stroke of diagonal brushes is broken
• The whole UI should be accessible through keyboard shortcuts. E.g. show a popup, execute a command in a popup menu, change a value in the context bar etc. This is something I thought before and it's related to macros Record macro/actions #475, as a macro could contain the execute of any UI element, these elements should generate recordable commands (same commands that could be triggered with keyboard shortcuts).

More to read here: https://twitter.com/cyangmou/status/818881183283023872 (Aseprite_1_1_13_Review.pdf)