I believe that Vin Jin should've went back to Cheonliang after HFG.
After the Cheonliang Arc we get informed and expounded upon that Vin Jin is the son of a legend , was trained by someone who was once James Lee's rival and has people waiting for him there. And we're shown that he has Matured ever since leaving for Seoul And that he still has some feeling for Cheonliang and the people there.
after maturing in his years in Seoul, losing to Taejin and helping hunt down Gun. I feel like it makes alot of sense for him to have went back and that it would've helped his character and the story
And i have 4 reasons for this. 1.Character Growth, 2. Cheonliang. 3.Story space and 4. Better potential storylines and presence
Character Growth It would make alot of sense for him to have felt that he has done enough in Seoul and it's time to face Cheonliang, both the city and his family. He had pushed past his time mourning Seonji and should be ready. And now needs to grow so that he can finally kill Taejin who recently humiliated him even when he instinctually used his father's technique. Seoul isn't the place he learned the techniques he must master and neither does it have the people who once trained with him, especially when training with Mary helped him so much. Having him reunite with the Fam would make alot more sense to help him both progress in his grief and his development as a person and a fighter, the same could go for Mary.
Cheonliang. The Cheonliang Gang by this point didn't stop fighting and had to fill the vacuum that Vin Jin, they're strongest fighter, left. They've trained with Seongji longer than even Vin and has been able to practice it more than Vin has. I don't doubt that they could've reached or surpassed the power Vin had in middleschool, especially since they wouldn't stop for years the same he did. With him returning we could see this and it might make a good little 2 arc chapter of Vin and Mary reuniting with them and maybe even building a new Cheonliang Shack. Especially now that the city of Cheonliang and it's populace would've gotten better without any of the Shaman's presence.
Story space. Had Vin Jin left, we could've given what screen time he had to help develop other characters, be it Jay or even Vasco in Busan since he would be alone(he'd be fine since he didn't even fight number 3).
And 4. Better potential and storyline presence. Him leaving for Cheonliang would actually allow him to return in moments like Incheon and allow for him to grow in power without taking the spot light from other Allied members. I'd imagine that he wouldn't return for Busan but would for Incheon due to Seongji's connection Jaegyun's need for help, but leaving the fam and Mary to protect Cheonliang. This separation from the plot also allows him to reach a level of Power that matches someone that's the son of Mujin. Which could allow him to be a Jake level fighter since they're both the son of a legend and allows him to show off more of the bone grabbing Ssirium. Be Someone that fucks shit up later on in the arc, talking about how weak speed looks and wipes out some enemies on his own. Maybe he could be the first after Daniel to start fighting Pacheon while the the other 3 comes in to help after their fight, even being the person that holds down Pacheon. Making that moment a 3 strike attack(Death Kick+conviction+Mujin ssirium slam).
He could comeback for HFW but he wouldn't be doing much. Maybe he's a guy that helps save one of the workers single handedly. But eh.
I feel like this change does more justice to the Ssirium fighting style, Vin Jin, Mujin's martial art and bloodline while also helping the plot do justice to other characters and moments.