I love ACF, and think this would be a great improvement!
So often I want to create a group of fields that are repeated in a variety of contexts and sometimes multiple times within those contexts.
Allowing us to easily pull in groups of fields as a single element would be very powerful. You could create an address group with all the required street, city, state, etc. fields and pull it in as needed. You could create a flexible content field that works with your various front end components and allow it to be added as needed within other field groups.
There are a lot of possiblities