I'm needing something as well. I have a movie review blog and am categorizing movies by Actor, Writer and Director. I was wanting to include Producers and Genre, but I need a better way to manage them first.
I have parent categories for the main types and then and making actual actors, writers, etc, children of their type. The problem is when I add new people to a post I have to go to my category manager and try to find the new ones. Once I find them I have to manually edit every single one and select its parent.
If there were a way to check several categories and make them all children of one parent that would be awesome. Or, is there a way to make a new category automatically a child from the Write-Post screen?
If it would help to see what I want, here's my blog. The end result is I want people to be able to look up movies by actor, director, etc. That's why I don't think a tag cloud would work. I also just started categorizing them and am about 10% finished so I have a LOT more to add.