How about a network graph for groups/categories?
When working with large databases it becomes hard to keep the overview of your references with the tree view of groups.
A network graph or graph with bubbles and subbubbles that shows clusters with group names would be great. In each group could then be the citationkeys you might be able to klick. To insert a reference to a new group could be done by drag and drop into a new group bubble.
Nice. I would like to add to your idea: The more entries a group has, the bigger the bubble. Let gravity pull groups closer together (or further away respectively).
Also, in the long run, having nice graphs, diagrams and charts that allow users to dig into bibliographic data and to draw conclusions from analyzing their data would be a nice thing to have, albeit I have to say that this might be a dream for now, unless somebody or a few people suddenly take a strong interest in such endeavors. For now, I would be happy enough, if the standard groups feature would work as expected T.T On GitHub you can see (check out the label “groups”) that currently it is the feature with the most open issues. Of course quantity says nothing about the quality of such issues, but you get the drift…That goes to say that groups are not entirely unusable. They work. One can do a lot of things with the groups feature, but there may be some hidden pitfalls.