It woud be very useful to allow for sub-groupings in the subject bibliography. It has always allowed for grouping by multiple fields, but all at the top level. Choosing multiple fields to group on and not allowing for one of them to be sub-grouped to the other makes it kind of a useless option for me. I have never used it. But if I could sub-group then I would use that all of the time.
At least to two levels would be great (more would be better).