I think the ability to add a custom spreadsheet as a custom field type. For example, when looking after IT for a company whereby you have passwords for many different devices, services, etc, it would be better to have these in a custom table rather than having to create a custom field type, and a whole bunch of custom fields for each customer.

I would then imagine that it would be editable in a very basic WYSIWYG editor, perhaps a cut down version of Google Sheets?

Just a quick and easy way to store data specific to that customer, as opposed to using the notes section where deletion is a bit too easy.