I really love Assassins Creed game to be set in East Asia,,whether it is in China or in my mainland, Japan.
I think the fact that Ghost of Tsushima already used Japan, should not hold off Ubisoft to use Japan again, but in different setting. If Ghost of Tsushima set in Mongol Invasion during Kamakura bakufu, then Assassins Creed could use the more ancient era like Heian era or Heike era, or my personal choice is Meiji Revolution era (this is the times where American fleet made its first arrival on Tokyo (at that time still named Edo) Bay and political figures both from shogunate and foreign ambassador being assasinated right and left by hitokiri/assassins).
If Japan is indeed being picked in the future by Ubisoft, I cannot think any more better setting for Assassins Creed game than Meiji Revolution era (the most famous non fiction samurai assassins (Okada Izo, Kawakani Gensai, and Shinsengumi samurai group) and also fiction (Kenshin) came from this era.
If China, I agree..the Three Kingdom era is the best choice, where we can choose as assassins from Wu, Shu, and Wei empire.
The potential is limitless if Ubisoft pick Japan or China (as the most rich historical countries in the world) as next setting.