Simulate color blindness for accessibility
It would be nice to be able to quickly check my work for accessibility. A button group/panel that lets me switch between protanopia, deuteranopia, tritanopia, and monochromacy would be really useful. Arguably, a monochromatic view is useful outside of accessibility, allowing one to check the tonal or value relationships in a design. This feature is not only applicable in Illustrator but across many of your products.
Egor Chistyakov commented
Constantine, View > Proof Setup has what you want, except for the tritanopia and monochromatic (which is not as simple, actually, because there are several ways of sucking the color out). Voted for these.