We have thought about doing this one a few times. Realistically it will probably come after users are able to make their own decks, as everyone will be able to put their own tags on things from among a list of common tags that we create. The main reason why we will probably do it this way is due to it being highly likely that different people will tag things in different ways, so this will be the easiest to get tags that the majority of people agree on.
For example, with country names, some people may put them in politics, while others put them in geography, while others still may put them in dedicated katakana decks. If we let people tag everything themselves, we’ll end up with categories that the majority of people at least somewhat agree on.