If I would watch a movie in which mythology plays a(n important) role, and if I would be wondering about the mythological accuracy of the storyline or of plot elements in the movie, I would certainly use a site called mythology.stackexchange to look for answers.
So I think that questions involving recent media, be it games, books or movies, are on-topic, as long as they actually refer to mythology — we do not want to encourage in-depth discussions of fantasy-world realities of WoT or GoT here.
Now, the question is if every book, film or game that raises valid mythology questions should be allowed its own tag...
There are a multitude of such new stories and we risk getting a tag-overflow if every mythology-related book, game or movie gets its own tag, so my initial idea would be to think about one or some tags that encompass all of these.
Maybe a single tag modern-fiction could do the trick?
I do suspect that eventually, when a new movie like Troy or a game like Age of Mythology comes out, users will try to create tags for those, and I'm not sure it's always a bad idea to try and avoid them altogether. After all, we want people to find those questions when they want to know if Troy was mythologically accurate. But that would be a question of keeping up-to-date with current affairs, I guess.
To avoid a modern-fiction becoming too broad a catch-all for too many different subjects, we could allow seperate tags for popular fiction that we expect to attract multiple questions. So a movie like Troy, especially when it is new, would deserve its own tag, but an obscure translation of a privately published short story loosely based on a mythological theme does not.