The Wei-Han-Chu alliance is my prediction of how Hara will play out the Han campaign that historically is stated to be an easy campaign, but I am predicting Hara will make it a huge grand one, due to it being Tou's 1st major campaign, and the 1st state that serves as the gateway to Chu & Wei will fall.
This is the last thing we know from Han, they wanted to establish an alliance with Chu in order to make sure they can defend against the Qin. And Han is already allied with Zhao right now. So if the the alliance with Chu goes planned, they would be allied with Zhao & Chu right now.
What we also know is that by the time the campaign of Han starts in 230, the Wei-Qin alliance will have ended. Wei will be back to goin against Qin by then as well. So essentially bt the time of the invasion of Han happens, there will be no "allies" of Qin like there is now with Wei. And as Riboku already established in the meeting with Ryofui, Han serves as a crucial defensive barrier to Zhao & Wei from being conquered and it's the first state that would need to be removed were Qin to unify the lands (and as we know it is the very first state to fall).
Assuming Han has secured a Chu alliance (I don't see any reason for Karin to deny it). Normally by the time the Han invasion happens in 230 BC, Qin would logically (within manga confines) be going up against the forces of Han-Chu-Wei-Zhao. BUT we know that Zhao suffers major losses in the next war against Qin in 232. So it is possible Zhao, who is an ally to Han, will not participate in that campaign.
Ultimatly, based on what the manga has established so far. When that Han campaign happens, my prediction is that we will be seeing two of the following scenarios:
Qin vs combined forces of Wei/Chu/Han
or
Qin vs combined forces of Wei/Chu/Han/Zhao
And tbf, Han would logically be the "easiest state" due to it being the smallest stated, however, we don't have any details historically of how those battles played out. So Hara has free liberties in making it the hardest war so far for 1 of the 6 states to finally be conquered by the Qin. Hardest war but the easiest campaign (since all other states take multiple wars to fall).
So it's just my prediction based on historical & manga information.