This is not really a suggestion for any one specific tool, but rather some specific things that I think are a part of almost every pixel artist’s workflow. Because they are so common, and repeated so often, they end up being busy work that ends up taking up like 30% of the creation time because there’s currently no shortcut for them that is viable.
This image illustrates three of the “busywork” aspects.
The first image is how many sprites might look at an early stage. Lines are all one color. In the second image, lines are replaced with colors. This is generally done with the brush, and based on the surrounding colors. You can do it very quickly with eyedropper and hotkeys, but it’s still not as fast as it can be. For this I think there should be a tool that allows you to paint “across” lines, changing their color. You can do this by selecting with the wand, but it’s a bit annoying to select the lines first, and the selection lines are a bit distracting.
In the third step, I added smoothing. This is where you use a mid-tone between colors to soften them. It’s a bit like antialiasing, but only one color. Currently there isn’t really a way to do this quickly, and so I almost never do it.
The third thing is changing the color of the outline based on the color within it. This would probably be the easiest thing to automate with a filter, however it would still be ideal to be able to do this selectively by brushing “across” lines.
All of these things may seem very specific, but I do pixel art every day, for hours upon hours, and so the ability to speed up one or any of these tasks would make a massive difference. Not only that, but automating or making tools for these tasks would make common pixel art techniques instantly accessible to beginners who may not know how to do them. It’s sort of like how Photoshop’s “content aware” filter instantly removed a ton of busywork for a commonly repeated task. Literally if it cost me $500 to have features like this, it would be worth it within like 2 weeks of using them given how much time it would save.