Currently, you cannot creatie a custom brush that utilizes adjustable hardness, even with a feathered edge. This significantly limits the usability for flexability of custom brushes and confines an artists's creativity. I would like to see the hardness slider become available for custom brushes that have a feathered edge.
Allowing for custom brushes to have a hardness slider would be extremely useful. There are many instances where, in creating a custom brush, being able to adjust the hardness would make the use for the brush singificantly more versatile and tre-to-life.