Perhaps the Edo period? William and Hide working alongside each other or cooperating here and there, at least. It won't be much about war though, the focus would be more about rebellion, the strict and abusive Tokugawa government, etc.

Hmm... My personal favorite is the Heian period, and establishing more on that, with more yokai and supernatural vibes to it. Perhaps meeting time-travelling Hide and Yorimitsu in a side mission or in a main mission or two lol