Hi,

I believe adding a tab where developers can add custom code (network + individual sites) would be a great addition.
Many developers still either functions.php, edit header section to achieve this.
A simple use case is adding google analytics code to the footer, but as you can see the power it brings along is huge.

Functionally similar plugins are: https://wordpress.org/plugins/custom-css-js/ and https://wordpress.org/plugins/custom-css-editor/

Disclaimer: Even tough I do use one fo the above mentioned, I am not here to promote them.

Thanks,
Sudarshan