Although I was hoping for a game about the Dark Age or even ancient world, I think Paradox Interactive shows an open-mind with this game. Japan is not a region so covered by their games, and even DW seems to not going far enough in history. So, a game about that is not my first choice, but if the mechanic seems interesting to me, I'll probably buy it.
Someone said that before playing EU3, he didn't know very much about Renaissance Europe. I can say the same thing about Victorian era, which is really interesting since I tried Victoria (I and II). Maybe this game can open my mind about Japan, but after that, PI have to do at some point a game about Middle East (given the current events, that would have been awesome to remind us what wonderful things they did in the past), something about India, Ancient World, and why not Pre-colombian America?
Well, there is room in history to make many, many, many new games. Don't be bothered by the critics, Paradox, you are just fine. (but maybe you should do more betatesting before the release of some games. This is what you are doing with CK2, in fact, so thank you).