Has there been any interest in having a separate layer format for watercolor effects, as in:
i) edge drying effect (in real watercolor this shows as a dark line around the wet area as it dries).
ii) diffusion or wet-in-wet blending (as when two liquid areas come into contact) .
iii) Granularity (a random particulate effect for some pigments).
Working from a brush design would be much more difficult than making an entire layer that is reserved for these effects. The layer could be kept separate and have a drying rate, or an instant dry/keep wet button.

The usual paradigm for paint in Krita and other paint programs is oil and gouache and acrylic, but watercolor would be exciting.

Ah! This actually used to be a thing, but the drying-filter started interfering in nasty manners with the undo system. It has been since removed, and while something like it is amongst the GSoC ideas, it is rather low priority.

I managed to get something working here: https://www.youtube.com/watch?v=c-LX4_fwnqA using a mean-removal filter for sharpened edges and the multiply blending mode for realistic colour mixing.
It's not quite the same thing of course, but the look gets close.