I see a problem =/. If there are only 13 + 1 episodes left, then how would they be able to come out with a K-ON! Guitar Hero game? The way I see it, they could only have the opening song, ending song (awesome), Fuwa Fuwa Time (the one they played at the festival), and possibly a couple more till the "end" of the series. Even if they did come out with songs for every upcoming episode (very unlikely) they would still have under 10 songs.
Right now, they are probably seeing how popular K-ON! is in order to see if the author of the manga should continue or something.
Also, MUGI <3