I've just stumbled upon a feature video of Paintstorm Studio.

There are some pretty nifty features. I especially like the Parameter blending to the Perspective feature at 0:50

@ iamcreasy,

Krita already has this feature. you have to just tick perspective check box for size parameter in the brush editor, and then whenever you draw on a perspective grid it blends in perspective.

Below is a quick and dirty screenshot :

Nice, didnt know that this exists.
But setting up a perspective to cover the sheet is very uncomfortable.
The perspective grid need to be infinite too or activate infinite ruler for that feature please.

This is on the todo, but I haven't been able to work on assistants since the 2.9 release.