Currently you cannot modify legend text in Insights. For example, if you had a choropleth map with 3 classes that you intend to represent High, Medium and Low, the legend would only ever show the numerical breakpoints. There is no way to change it to more descriptive text of the classes.
Also, more control over colors across the platform is needed. With quantitative data, you can select a color ramp, which is nice, but you can't reverse it or create a custom color ramp. With categorical data, you cannot select the colors at all! You are just stuck using the random color defaults.