If you guys setup the customization, but not the UI I would be okay with going into a text file to customize it early.
Outside of that it would be amazing if you could use or at least have the ability to use the Paint Tool Sai shortcuts that revolve around control, alt, shift, and spacebar. These use those keys+lmb to give you basic, but frequently used tools. Things like zoom in, zoom out, eyedropper, hand, rotate, and most importantly change brush size.