This request is very close to being fulfilled AND technically is complete via the OTX and Morevna releases.
As for creating custom brushes (both vector and bitmap) we have several methods available to us:
MyPaint Brushes (for use with bitmap/raster images)
In addition to editing one of the many MyPaint brushes supplied the MyPaint application itself can be used to edit and create brushes.
Of note. Almost all of the variables used in the MyPaint application are available via the Settings tab in the Style Editor.
The primary things lacking in the OT implementation are the icon editor (useful for creating custom icons for our brushes) and the ability to add those new brushes easily to the MyPaint LIbrary. The MyPaint application has a means to automatically update the library with the new brush.
Vector brushes
Perhaps the most straightforward way to create a new vector brush for use in OpenToonz is to use OpenToonz to navigate to the vector brush library, open a current brush, edit that brush and then save it back in the folder under a new name.
Other brushes (standard bitmap)
While the MyPaint brushes are likely to be the desired approach for creating new brushes another way to create bitmap brushes would be to edit a current brush and save it under new name in the appropriate directory.
We need to add a few links to this topic that relate to creation and use of pens and brushes in OpenToonz.
Here's one on usage in OpenToonz by the developer of that functionality:
Here is a series of video tutorials on use of MyPaint to create new brushes: