Recent announcement by Yorii Film Commission regarding Kamen Rider Zi-O the Movie's Extras Casting had revealed a major detail regarding the movie. According to the description of the extras casting, it had revealed that they are going to play the role of soldiers in the Sengoku War. By Sengoku, it only reminds me of Kamen Rider Gaim but he might not be related to the movie.
Since Kamen Rider Zi-O can travel through time and might end up in the Sengoku era. And think about this, Kings existed in this era (I mean in Large Scale compared to the present. LOL) so it might have some connection to Sougo becoming King.
P.S. Gaim has nothing to do with the movie. The detail just reminded me of Kachidoki Arms so used it.