I think there is a lot of potential for the culture to gain more events.

One could be little interactions with japan, like for example asking for the Japanese Emperor's recognition. Sorta like how the coronation works. You would need some gold and prestige to gain an imperial edict recognizing you as Shogun of the West, gaining a trait after that. Each new ruler would have to do the same again.

It wouldnt need to be a series of events like the coronation is, but I think it could be an interesting addition and gimmick. Not caring about requesting an edict could negatively impact same dinasty opinion or perhaps vassal opinion.

The Annan Protectorate could be incorporated in this mechanic and forming it could have the draw back of angering china. Making the New Japan would break away from the homeland and the need to be recognized.

Speaking of China, Chinese relation with Japan were those of trade, however once the mongols took over must of China, the Yuan Empire had a rough relation with them, leading to the 2 failed invasions of the islands in 1274 and 1281. You could add a event pop-up for those years explaining the mongol failure and in both cases you could lose grace or gain grace with the chinese emperor depending if he is mongol or chinese. It will mostly likely be Mongol, but idk if its a random chance of a programmed event in which the mongols always take over china. Again, this was a major event in japanese history and I think it would be neat to have it included in this mod.

Another thing I had in mind was the relation between japan and Buddhism. Buddhism played a big role in japan and perhaps a Japanese immigration or relief force could be motivated by a request of aid by indian buddhist against muslim expansion in the region. It could be a good base for a aditional scenario and even event. It would be entirely fictional, but thats what this game is about anyway.

I think thats all I have for now.