Probably one of the features I find myself missing the most from other pixel art applications is a way to flip the current brush, or rotate it by 90 degrees. While of limited/no use for circle brushes, this would function mainly with custom brushes and allow flipping the Square/Line brushes.
While the basic Square and Line brush types have a Brush Angle option, this would be a temporary change that reverts back when swapping to a different tool or brush, or a Brush Tool Modifer.
Some might wish to keep those orientation changes between tool and brush swaps, so that could be an option. Where the brush remembers/keeps the new orientation with a way to restore it to the default orientation.