It would be lovely if Aseprite could adopt this behavior from Photoshop, Affinity, Photopea and other apps.
In these programs, if you hold the modifier while dragging horizontally, brush size is adjusted; vertical dragging changes hardness of the brush.
Since Aseprite brushes don’t have hardness, other parameters could be chosen for the vertical axis—like angle, cycling through saved brushes even shading mode.
On Windows, Photoshop has this mapped to alt—right-click, but alt-clicking is reserved for colorpicker in Aseprite.
That’s why i would suggest the Mac shortcut instead, which is ctrl—alt—left-click.
I think everything about this should be adjustable, including the parameters for each axis, so people like myself who have it in their muscle memory from other apps can use it the same way in Aseprite.