Drying time for paint could be interesting but a great one for me for added realism would be the option to have a finite amount of paint on the brush. So with the oils it mixes in until essentially you just have a blender brush rather than the original colour coming back in. A little refill paint pot next to the water for cleaning the brush would be perfect. It would work in conjunction with the amount of loading parameter (and possible the deposit feature on the custom brush). It's always been my one niggle really, as I use the oil brush a lot, but when the colour starts to refresh it interrupts the natural flow of working.

I was also playing with the gloop pen. It's interesting for creating stains and blotches but generally a bit slow in response. But if the way it blooms when held down could be combined with the watercolour tool, that could be really interesting, especially when adding in the wet and dry parameters. In reverse - add the wetness/colour pickup ability to the gloop pen so that it can blend rather than just layer up.

Not asking for much