I would not yet discount the possibility of Qin pulling a Great General from an entirely separate state just yet. We’ve seen other Great Generals like Gekishin flirt with the ideas of switching states lol.
Not that I think this is likely to happen, if I had to give my predictions in order of likelihood:
1. Mouten
2. Ouhon
3. Shouheikun (will explain more in the chapter thread)
4. Shin
5. An outside GG
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Koshou though was still a proper general in the sense that he was actively involved in campaigns.
I don't think SHK could be the six GG given he is already the Military Minister and Chancellor of the right/left. If Hara wanted him to part of that group he would have written him Karin/Riboku where he is involved in both grand table top plans and actively participating in the fields during the actual wars.
The problem with this viewpoint is that Shouheikun’s position is made completely unnecessary by the introduction of the GG system. Qin doesn’t need a head of military affairs when each GG is capable of organizing armies independent of Kanyou.
So unless Shouheikun is immediately promoted to Chancellor of State for Qin, I can’t see him remaining at his current rank.