This may be better suited to an addon plugin, but it would be nice if we could have a Code Editor custom field widget (Code Mirror, maybe).
I have several CPTs where I attach CSS and I’m using the standard textarea right now, but having a code editor with line numbers, coloring, etc.. would be dang useful.
There was/is an add on, but it hasn’t been maintained or updated to work with ACF5. https://wordpress.org/plugins/advanced-custom-fields-code-area-field/ It’s also available on GitHub https://github.com/taylormsj/acf-code_area-field.
I imagine keeping up with ACF and Code Mirror was more than the developer could handle.