I'm looking forward to sponges. Sponges are typically just dabbed, but can be stroked too.
A grayscale image can represent the texture of the whole sponge, and you could have a drawer full of different sponges: kitchen sponge, square, ball, rough-ripped, sea sponge. The grayscale shows the holey texture of the material. The outline should match the silhouette of the sponge (all but one gray level being the inside).
At the lightest presses, the outmost sponge pixels (one extreme gray level) touch the outmost paper grain areas. As you press, more sponge meets more paper grain. At middle pressure, the whole silhouette of the sponge (255 levels) is able to touch the paper's outmost grain while the sponge is really saturating the valleys of the paper grain. At nearly full pressure, the silhouette of the sponge is finally saturating the deepest grain. At full pressure, "squirt" a rim of paint around the edges of the sponge silhouette.
When pulling a sponge away, leave peaks of wet paint structure at any local maxima in the sponge structure. Algorithmically, this is easy: just iterate back through the decreasing pressure levels, but instead of meeting sponge to paper grain, just meet the sponge to the already-applied paint. You'll build paint on paint in the right spots automatically.
It would be nice to have a control to rotate the sponge. Choices I can imagine being useful: always aligned with the screen (rotate the canvas for other angles); always align with the direction of drag (as brushes work); align with the direction of drag only when off the canvas; vary alignment randomly on each new press.