AHC: No Sengoku Era:

[Snake Featherston]

In medieval times Japan had about a century's worth of sustained, endemic warfare that Japanese sources call the Sengoku Era. The challenge, should you choose to accept it, is to avert the Sengoku period. What kind of ramifications would actually exist, if any?

 

[St. Just]

It depends on how you avert the Sengoku. If the nobles are cowed and the Ashikaga stay capable, then it is Ashikaga world. If the nobles are crushed and the Ashikaga still fall, then the political situation of Japan is up in the air. A resurgent Emperor would be interesting.

The lack of Sakoku, the lack of massive amounts of warfare, the butterflying of Toyotomi, Tokugawa, Nobunaga and others, would be massive for Japan.