I don't think Ousen will end up "neutralized". I really see him being a major antagonist at some point. Qin and Sei will pay dearly for giving him so much power. I think we'll see him recruit power generals from each state that has fallen and secretly imass a lot of loyalty.
I'm curious about your theory though, would like to hear it.
I actually see the final war of Kingdom being Shin vs Ousen as a callback to Duke Hyou vs Gokei.
An Ou Family Rebellion could occur but would have to be stated to be suppressed from the chronicles. imo it's probably the most exciting premise Kingdom could go for, but I think it's too massive of a departure from the accepted history for Hara to go for it.
I'll save the full theory for another post but: Ou Sen cannot become king unless he usurps Sei or strikes out and carves a new kingdom of his own - which would likely have to be Ou lands, i.e. within Qin's borders and therefore indefensible considering Tou and Yo Tan Wa have already declared their intentions in the event of rebellion, and Mou Bu is likely oppose him on his honour as First Great alone, if not for family interest, debt to Ou Ki, and consideration for his son position as one the king's chosen.
Ou Sen cannot hope to win against these odds with only Ou Hon at his side.
There are only two paths to secure victory here, and theoretically he could travel both. In recruiting Mou Ten he stands to gain a rare asset and the possibility to at least neutralise Mou Bu.
If he can recruit Shin, a man capable of slaying Mou Bu, and the likes of his powerful army that includes equal tier rare talent Kyou Kai and Shou Hei Kun's student Ka Ryo Ten. He also gets potential levers to manipulate with Tou and Yo Tan Wa through their relationship with Shin and trust in his judgment.
I don't see either working out for Ou Sen, so I don't see him ever actually taking up arms against the throne. I think his defeat will come through his failure to convince Sei's allies to turn against him.