This is one of those quality of life things, not a functional matter. I see this in two phases:
Phase 1
Able to control font, text size, and color from inside the editor while working with text, title, and subtitle fields in the builder, without having to deploy any CSS.
Phase 2
Able to control font, text size, and color from inside the editor while working with any text related fields at all in the builder, without having to deploy any CSS. For example, I see this as a little menu near the “Display as” configuration where you can select the color, font, and text size.