In order of importance to me.
- Individual button colors (edit: I found the Custom Variant option, so good here)
- Separator. A vertical line that would separate groups of buttons.
- Alignment. Specifically each button should be able to either left, right (and maybe center) align so you can end up with groups of buttons. I used this extensively in other UI libraries in the past. I know I can do this with all buttons and I could use two Button Groups. But that can be less space efficient.
- Maintain space when hidden for individual buttons.