Feature: Value Propagate to a neighborhood of pixels
Currently, the Value Propagate operation is only able to propagate to adjacent pixels. An option to propagate to a neighborhood of pixels in a circle, square, or diamond of a given radius would offer a number of benefits.
Particularly, this would offer feature parity with Photoshop's "Minimum" and "Maximum" filters, which are useful mostly for modifying layer masks, but also have some artistic applications, such as making line-art bolder or thinner. I've also seen it used to produce a bokeh effect in image retouching.
A Value Propagate with radius could also be potentially chained into the Drop Shadow op to allow growing/shrinking the shadow, which itself has a wealth of uses.
Radius should preferably allow decimal point values, rather than just integers. Whether the edges of a circular neighborhood should be anti-aliased could also be worth adding as a parameter.