It would be nice to have different types of brushes, like real brushes. You can kind of simulate it now, but it would be better if you could choose a fan brush, or a flat brush, or a pointed brush, etc. (or a big 2" or 3" or larger type paint brush)
And also being able to select soft, firm, or hard bristles would help. And the thickness of the brush (and width), not just the "size". And, I suppose, the bristle density would also be needed.
I'm not sure how you could do this, but it's nice that the brush starts out pointed (sometimes, a flat brush (usually) wouldn't do that), but as far as I can tell, you can't make it end up pointed. It's always flat width at the end of the stroke, unless you curve the stroke and it rotates the brush. So, if you could somehow add a pointed end stroke, that would be nice. (to do, as merely one example, leaves) I guess you can kind of simulate that now by stroking halfway, and then lift and go to the end and stroking back.
Maybe you could have options to make it start out pointed and end 'flat', start out flat and end pointed, or start and end pointed or flat. This might require a "stroke length" setting, or something like that. To simulate a pointed brush where you want it to start out 'full' and then draw it to an increasingly pointed tip (like for drawing branches on a tree), you could have a setting that made this occur, as well.
I'm using a mouse, so maybe it would require (or be easier) to do some of these things with a tablet pen. But you could also make them work, more or less, with a mouse, too.
(one way to do it would be to keep painting after you have taken your finger off of the mouse button, and that's when you start the pointed end stroke, or other features -- on PC two-button mice, the left could be 'normal painting' and the right could be the other mode(s) that keep painting after the button is released -- with a stationary reset time (sound notified?) when you don't want it to keep painting when you start moving the mouse again -- that is, to signify 'this is the end of the stroke')
Regarding pointed ends to strokes, you can achieve this by slowly lowering your pressure at the end of the stroke. This presumes you are using a pen and tablet, not a mouse.