People need to be able to go gradually.
Yeah, tried that before. See [1] (Template:Tag). But that would be quite the kludge. There are plenty of ways to change the category system. What should come first is either a dicatorial decree or - if it must - a vote/!vote for switching systems.
If that is decided a bot could easily be run to write a flattened category list onto every category page. That list would have to be manually reviewed for goofs like Aryeh pointed out. Furthermore we'd have to blacklist every category that does not describe a singular concept. Can be done using templates that are manually set and read out by bots.
(abbreviated) example: Category:Churches in Guernsey
would get a template on its category page with all the categories that occur somewhere above "Category:Churches in Guernsey" in the tree: Category:Churches in Normandy Category:Religious_buildings_in_Guernsey Category:Religious buildings in Normandy Category:Buildings in Guernsey Category:Guernsey Category:Religion in Guernsey Category:Buildings in Normandy Category:Architecture of Normandy Category:Normandy Category:Architecture of Europe Category:Provinces of France Category:Provinces Category:History of France Category:History Category:History of Europe by country Category:France ...
A ginormous list. However every blacklisted category could already be filtered out! Leaving us with Category:Guernsey Category:Normandy Category:Provinces Category:History Category:France ...
Well Category:Churches better be in there somewhere ;-). Anyhow that list will be much shorter now, and users can weed out nonsense categories like the abstract Category:History and Category:Provinces quickly. A bot could then recategorize all images in the reviewed category.
[1] http://commons.wikimedia.org/w/index.php?title=Special:Undelete&target=T...