While I was looking for infos for deciding to buy the game or not, I found a link to this mod which seems interesting.
So if you bought the game and even if the mod isn't updated anymore, may be you should take a look ;)
(but I can't guarantee it will work with GOG's version)

The mod can be downloaded with the red link of the first post here : Monumenta Iaponiae Historica.
And here is an overview of the mod (but you can find a shorter and actualized version on the download page of the mod).

In short, goals of the mod are :
I. Expand the timeline from 1180 to 1600
II. Expand historical data
III. Expand game mechanics

Hope it could be useful to some people :)