Well, Topic Icons already exist, though they're not very useful and only seem to be enabled in certain subforums. I was able to use them in the RO subforum, like this:

...but I can't figure out how to use them anywhere else, so I don't know. They appear floating over the topic image, like so:

...so editing them to contain words isn't really an option unless we can smush the words down to a really tiny space. However, I have seen forums that use thread tags, so it could potentially be a thing.
Alternatively, we could just have a mandatory title tagging system if we really want to. But I think the subforums within Stories exist for a reason, and Serious Discussion threads are generally pretty self-explanatory. So I don't know. I guess it would be cool to filter stuff by tag, though.