One of the more time consuming techniques used in pixel art is selective anti-aliasing, as seen here,
The 2nd and 3rd images are how it would look if you use it on the inside of a shape vs the outside.
I dont know how this would be accomplished, however even a slightly rough version of it would speed up a lot of shading work. I figure it would be a fill sub tool, however it could also just be an effect.
Options for the tool could be what directions to alias in, since sometimes you only want one, as well as which color to use. The main thing is, compared to anti aliasing in other software; the ability to only use one color. The fastest method to get a similar outcome seems to be using a smoothing/blur tool in other software, and then indexing the result or replacing the colors after the fact. (however that process is pretty slow.)