It would be neat if it would be possible to attach more than one set of product category fields to a product group. Some times it's nice to have one set with general settings (used by all your groups) and one with specific fields for that particular group.
Imagine if you will my new shop "Plushies and Arms R'US". It's a pretty neat little thing, with a complex gallery with pictures and medias in all different sizes, so I create custom fields for all these media options. I collect these in a set called "General media".
My shop sells plushies, which may have some very specific options. Softness, cuteness factor and so on. I create a set called "Plushie settings".
However, I also sell doomsday devices. They should also exist in my VeryComplexGallery™, so they need the aforementioned media set. Like the plushies, they also have some very specific options (devastation in megaton, whether or not there is a waiting period for mad scientists etc.).
Sadly, I cannot apply the "general media" category to both groups, as well as their specific categories. I must therefore either make one category with all fields possibly used in my shop, or create two categories with the media fields plus their specific fields.
I do know this is technically possible by combining the product category fields with field groups. However, the documentation states that field groups will be removed in DW8 (it is currently still included, but I assume it is considered deprecated).
My proposal is therefore this: the list in which you assign product category fields to a product group should not be a select box, but a multiselect box instead, making it possible to assign more than one set of product category fields to one product group.