Not sure if this has already been suggested, or indeed how plausible it is but here goes:
I understand that an entire post can be created with a GF form (with custom fields) and that there are lots of plugins out there that try to make the process of playing with custom fields a little easier/pretty for users (not all succeed).
I can only imagine how sweet it would be to be able to create a form in GF and then have it appear on my edit post/page screen as a means of controlling input of custom fields. I'm guessing that the form would have to not include a submit button and use the save post/page button instead (I'm not even sure that you can hook this in?).
This along with upcoming WP3 features such as custom content types could be a fantastic combo. It would certainly make my life a lot easier. I could see lots of really smart ways that GF forms could appear in the admin of a WP site.
I'm pretty new to GF - if I've just described something that you can already do (and I haven't realised) then I'd really appreciate a few tips. Cheers.