I do think Moubu will slay Shouheikun, but SHK has buildup with Shin as well. He has always noticed and held the Hi Shin unit in regard, relied on them to help with the State of Ai incident, and even gave Shin an opportunity to kill Riboku.
https://www.reddit.com/r/Kingdom/comments/bspr7v
Shouheikun may be the one who creates the overarching strategy that Chu uses against Qin while Kouen takes the role as the commander in chief who acts it out.
For all we know, Shouheikun might personally kill an important commander of Shin in front of him, or maybe even personally duel him during the attack.
Renpa may die to Shin before his defeat if it happens. Ouhon's campaign could be an opportunity for Shin to kill Renpa, and even the Han Invasion could since Raku'kan said that Han should ally with Chu. Though I'd prefer Qin to have to deal with Shouheikun, Kouen and Renpa.
and regarding the commanders, Sosui, Denyuu, Suugen, etc are likely screwed at least. Garo too maybe. Maybe a few of them will be Mouten's commanders.
I can see Kouen being Shin's arc opponent, but Shouheikun may end up being the main overarching antagonist of the Chu campaign.