Stability, compatibility
VR button & offset management /presets for various systems

Extend documentation & tutorials

Filters (paint, color correction,blur, sharpen....)

Layers
"Shells" (i.e. additional skydomes at lower scales with transparent or cutout parts that can be stacked for parallax effects. Probably billboards or partial skydomes as well. )

Brushtip generation/editing (Use Shaders to create dynamic brushtips on the fly. Based on base shapes or imported pictures: edge glows, outlines, falloff tweaks, color inversion, and the likes can be used for dynamic brush creation and tweaking, greatly reducing the number of brush source images that need to be 1. loaded 2. managed my the user)

Quick selection for brushes (basically an Item/skill quickslot bar)
Quick selection for camera bookmarks (scene Positions)
Different menus for quick tool access, as configurable as possible.
Fancy extended effect brushes

Constant interation of pretty much anything :)

Marketing/Publishing
Creation and care of documentation & tutorial videos
Ongoing packs of free content and example presets

Support more skydome/box formats
Avoid remaining distortion at the top

Get and evaluate user feedback and implement it as good as possible.