The magic eraser and color selector tools are great. However, these additions could make them even better.
Feature 1 - Choose color variations from color wheel. This is so you can choose the dominate version of the color as well as variants due to shadows or folds.
Feature 2 - Spill Level Control. This is to remove spillage of the removed color on the subject you want to keep.
Example: Final Cut Pro X Keying PlugIn
Insert image on timeline
Apply Keying Effect
Generally if green screen, it is instantly gone upon application of the plug in.
There are controls to control the matte, spill, and level of subtraction.
On my most recent project, my green screen hung like a curtain. So there were many imperfections. Thus the magic eraser was having a tough time. But even after getting the background out, I still had spillage to control. So my workflow was to throw the pic in FCPX, apply Keying, remove spill, export current frame as PNG. I could then work with the photo in Pixelmator Pro for remaining alterations.
What features would you like to see in Pixelmator?