Hello, really like your program! Especially brushes and interface, better than in any other drawing software I have seen. So here are my suggestions in order from most important to least important.
1.) Adjustment layers. Really simplify work with light and shadow for me.
2.) Hotkeys for selecting layer above or below current layer.
3.) Hotkeys for decreasing/increasing of brush size should change it for a fixed value. For example: if I press one time hotkey for increasing of brush size and one time for decreasing I should have the same brush size value as it was before changing it.
4.) Marking layers with color like in photoshop. Faster and more clearly too see than naming them.
5.) Color themes that separated from workspaces. And ability to import and export them would be nice too. Changing twelve colors manually from settings is not convenient.
6.) Some kind of snapping grid for custom panels.