I'm curious how Knights of Ren got a sub when there's loads more of other things out there more popular from a potential sub basis?
As for autotagging...intriguing idea. A forum is a large database, tags are simply a field in the database entry. They are somewhat different from description and name (etc) in that those contain a single entry whereas tags is a field with multiple entries. It would not be a rewrite in the slightest to add sort by tag. It would, though, result in multi-tagged entries showing up mutliple times. This would be more of an add-on or whatever this boards software calls it. I'm not advocating going that far, but if I worked on vbulletin, i'd be looking to see if such an add on existed or start tinkering on my own
But again, it comes back to the point of typo's, as long as you have to type in the entry, or you can, it's not effective because lightsaber != lightsabre. So, if you want to can the entire list of potential tag's - great, but it'll be a royal PITA to generate that list AND keep it up to day.
I'd like to think it COULD be made to work BOTH ways. Have 8 subforums (just a random number, don't panic) under Star Wars and have a checkbox or something at the top of the screen (basically a single check box) that says SHOW SUBS or DONT SHOW SUBS. When DONT SHOW is checked, you get a single star wars forum that shows all the topics under the Star Wars header, subs included. If SHOW SUBS is checked, you get a main star wars forum and the subs in their respective folders. It's close to the same principal as a global search returning information from mutliple forums. There IS a way to know that SubB is underneath/linked to MainB. So it is possible to write it to show all threads from MainB as well as all threads from SubB. Just an idea. Could also be applied up the line as well. For those who want it you could have it come back and list all new prop threads in single view.