I think Xenforo filters (the descriptor infront of a thread title) are different from their tags, so while it might not be possible to limit tags results to a specific subforum, it looks like it is possible to do that with the filters.
For example,
filtering by "Discussion" in the Fanfiction Discussion subforum brings up a different list of threads than
filtering by "Discussion" in the News & Politics forum.
That's the kind of thing I was imagining for replacing the Ideas/Previews/Talk subforums. I know that in the Suggestion Box forum on SV you're required to add one of these filters/prefaces, and I know that SV runs on Xenforo, so this has to be doable. It might be some add-on that they developed themselves, however, so I'm not sure how difficult it would be to get your hands on it/implement it.
I think
@daniel_gudman is right to be concerned about new thread creation becoming intimidating, but if it's just that they have to select one of three preset categories for their thread, I think new posters wouldn't find that too offputting.