Hello, I have few ideas for you.
1. more, more moooore keyboard shortcuts (best for everything! :-) - easier way to define shortcut with RM? Click on something (f.e. button of making selection from path - open dialog for defining shortcut f.e. Ctr+Enter on NPad)
2. filling with BG and FG color: Alt+Backspace, Ctrl+Backspace, fill only over existing pixels depend on its opacity Shift +Alt+Backspace, Shift + Ctrl+Backspace
3. increase /decrease brush size/hardness with mouse (like in PS or Krita Ctrl+Alt+drag RM)
4. set selection from layer and add selection from other layer with shift? it is possible to do it through quick mask, but it's not so comfortable.
5. in quick mask: gradient fill
6. quick mask displaying with invert colors and option to change color of QM
7. for last filter repeat with dialog window for its adjust (Ctrl+Alt+Shift+F)
8. BG and FG color in one column's tools panel
9. shortcuts for macros, without change tool after execution macro
10. option for toggling between tools with Shift, or disable toggling
11. moving selection while making (space), option for add selection (shift) instead of LM+RM
12. checkers when transparent BG (maybe wrong displaying white only for me?)
13. lock layers not only for transform and move, but for painting too
Have a good days!
Klababa