One feature that I know could save a lot of time is having the ability to include a field group within a different field group as a standard block.
For example, on many sites we have a button group that is duplicated multiple times based on where it appears on the site, could be within flexible sections, repeaters, or just within a list of fields on the page.
However, if down the road we wanted to update this, say add an icon option to the button, we have to edit multiple instances of this group, whereas if there was a way to create a set this as a field group, and include it as a reference.
When editing pages you would set the individual options for the relevant section where it is included.
What you describe is what a clone field is useful for. You have one field group that is edited and then is cloned into other field groups so that you only need to edit it in one place.
