1 bit “sieve” where the brush only draws on marked pixels on a 16x16 (or smaller) grid. A few basic preset patterns, the ability to save patterns, and shifting patterns would be great.
With a custom brush set to “pattern aligned to source”, the brush size is set. (unless you’re clever with select/fill but that’s tricky). A sieve makes drawing complex dithering patterns super easy.
Here’s a screenshot of the sieve tool in Graphx2:
