Kanki Army Returns
This theory is a long shot but it would fit the story well enough, Hara forshadow that a Kanki army could return soon enough for the last battle.
Its not a certain, maybe even a long shot but Hara has always given us clues or hints about certain events with Maron saying Kanki wanted us to be Mercenary army an not return to being bandits.
I believe that this wasnt just a spur of the moment but could be a contingency plan he had, Kanki knowing he was heading for a trap before maybe already knew a general who left to become a mercenary.
Its a fan made idea, but what if Kanki had a general who was as smart and ruthless as him, a second in command, what if Kanki had him join first before he meet the other bandit leader.
If Shin army gets ambushed and crushed at which the Mercenary Army arrives, defeat Riboku main army and crushing him in revenge for Kanki.
With Shin and the mercenay army he could give a massive blow to Riboku, Its could be testimony to Kanki genuis that he made a contingency if he died, Riboku might be defeated by Shin and Zhoa army collapse.
Well its a crazy theory anyway, what do you guys think, even without Kanki, A mercenary army would be needed to help push Zhoa and defeat them.