I remember that tags could now have a predetermined list, which would work just fine, e.g. type in your car make and, as the site owner, I would have a list of approved tags and the new thread requires only one tag (not more) to be added.
The other way was with custom fields - I'm aware they are available by user fields and I think there may also be post/thread custom fields? If that was the case, I could possibly create a required field as a dropdown and done.
If neither of these are practical solutions, I guess it's time for an add-on!