Currently, if you want to break up messages for a category, the bucket system requires all of the buckets to be in the same directory, which hurts performance when you get into the thousands of subdirectories under the same parent. How about something like php's "N;/path" in session.save_handler, where you can have it break up into groups N levels deep?

Also, right now, when you try to use a category name with a / in it, it appears incapable of creating the full path. Perhaps some analog to mkdir -p?