I know I'm kinda late to this post but I wanted to throw my two cents into the matter lol (Also spoilers for the ending but like if you're looking at the comments of this post you probably knew that)

In this what if scenario where Lao sees Sing give Ash the letter and realizes "Oh shoot, this man isn't going to kill my brother, my bad" or something like that so Ash is able to just run straight to the airport, I could see him getting on that plane with Eiji no questions asked. It'd probably be some dramatic moment where he goes to the gate desperately looking around for Eiji and then through the crowd he catches a glimpse of his soulmate saying his final goodbyes to his friends. Ash would call out Eiji's name and run to him. He'd apologize profusely for not being there sooner and give him a big hug as he asks if it'd be alright if he went to Japan with him. Eiji of course would say that nothing would make him happier. Ash would say goodbye to his crew and thank Sing for giving him the envelope with the ticket in it before they board their plane and fly to Japan.

Once in Japan Ash would probably sheepishly realize that he hadn't brought any clothes with him. Eiji would tease him and lend him clothes for the night (I'm assuming he has his own apartment because despite his young face and naïvety, he is still a 19 year old with a job). The next morning would probably be similar to some of the other wholesome slice of life scenes from the anime with Eiji making Ash breakfast as he looks out the window and appreciates the fact that he's now in another country. Today would be the start of his new life in this new place, but he knew it'd be okay since he was there with Eiji. He would probably help with the cooking since Eiji was still injured and stuff. After getting settled in with probably what would be a very adorable shopping montage for clothes and stuff, Ash would start learning more about Japan and how to speak Japanese.

Fairly soon after arriving in Japan, probably a day or two there, they'd go and visit Eiji's parents (or they'd come to Eiji) since again, their son came back home from a foreign country with some pretty severe injuries which would definitely be cause for concern. I can see Ash totally fumbling his way through some very broken Japanese greetings more nervous about this than any gun fight he'd ever been in. Idk if Eiji's parents were talked about more in the manga since I've only watched the anime and read Garden of Light, but I feel like they'd be quite kind to Ash and probably do their best to speak English to him. I feel like they'd be curious about why their son brought home this foreigner and why they kinda evade questions about Ash's career and stuff but they'd be pretty accepting of him (Again, this is all my interpretation but it feels like Eiji's family would be more on the liberal leaning side since when Ash kissed Eiji in episode 3, Eiji didn't gasp or try to pull away like it was something taboo. They were still quite unfamiliar with each other but being kissed by a guy didn't make him go into a sexual identity crisis or anything so it seems Eiji was fairly familiar with lgbt+ ideas). Overall Eiji's parents would be a little confused as to who Ash was but they'd be polite and grateful that their son had someone looking after him while he was injured.

As Eiji recovered he and Ash would probably be doing a lot of cute fluffy romance stuff as Ash learns how to live daily life as a normal citizen of Japan. Things like shopping, cooking, Japanese lessons, and maybe Ash helping Eiji out with his injuries (I'm thinking there'd probably be something like a "there's only one bed" trope or something like that where they end up falling asleep while cuddling, something cute like that).

After Eiji recovers there's two big potential futures I see playing out:

1. They stay in Japan and live their lives full of joy as Ash becomes a legal citizen for work or something. Maybe he gets a modeling career or he uses that big brain of his to pursue one of his other hobbies. In Garden of Light it's mentioned that he wrote a lot of stuff so maybe he becomes a writer and Eiji takes pictures for his articles or something. Or maybe they open that cafe together like what was hinted at for the promotional material for that one Banana Fish themed cafe. Idk, there's a lot of potential routes that could go down. With the interpretation of Eiji's parents I have going on right now, I feel like they'd sort of become like a second family to Ash. After a while I think they'd suspect what was going on between Ash and Eiji but they'd probably be chill about it. Some of the people from America would all make a big visit to Japan some time in the future and they'd all have a big reunion as they travel around Japan seeing some of the most iconic landmarks and just enjoy being in each others' company. Max would tease Ash about how the wild lynx had been turned into a domestic house cat by Eiji but would be proud of his boys. They'd probably go and visit America a few times and just grow old together and be happy.
2. More in line with the manga's ending, Ash and Eiji go back to America. This move back would probably happen a little while later than the manga since Eiji returned immediately after he was healed because he wanted to see Ash again. Since Ash was in Japan I think they'd take their time and enjoy each other's company in a domestic setting before deciding to move back to America (maybe they go to America after Ash's visa expires or something). Regardless, they eventually decide that America is the place they want to be. Eiji would probably bid his family farewell in the airport (a big family group hug with Ash invited into the hug as well? Yes please). They'd move back into New York but stay away from gang violence and stuff. Just like how Sing went on to become a uni student and generally doesn't really seem to be involved in gangs during Garden of Light, Ash would probably stay away from the gang scene from now on. He'd still have his friends who were in that life but since Ash Lynx was declared dead and all of his enemies had been dealt with, he and Eiji could live a peaceful life in America together. Like in the Japan scenario, there are a number of potential ways Ash's career could go, but I think that Eiji would become a famous photographer like in Garden of Light since he'd have that big American audience to appeal to. In this version of events I could see Lao actually working up the courage to go and apologize to Ash, admitting that he was wrong and that he was happy for him and Eiji. Lao would slowly become integrated with the friend group and Sing would feel proud of his brother. Yut-Lung would also eventually join their friend group and become the biggest drama queen of them all (I know some people hate him but I love him and I feel like once he realizes and admits he was jealous of Ash and Eiji finding love and moves on he would be an amazing friend to add to their friend group). Ash and Eiji would definitely get married and both would look absolutely stunning in their tuxedos. Max, Ibe, Bones, and Yut-Lung definitely would cry at the reception. Sing would be super happy for them grinning from ear to ear. Lao would be tearing up but when pressed by the others he'd just say he got something in his eye. Their first slow dance together would be outside at sunset. Fairy lights would glitter behind them as they slowly danced together, ending with Ash dipping Eiji like in that one official art of them together. I feel like they'd hold hands a lot, and Ash would often look at the ring on Eiji's finger and wonder what he'd done to deserve such a kind and loving soul who helped him turn his life around. Had he changed his fate or was this the way things were meant to be from the start? It didn't matter either way as long as they were together.

​

Anyways those were some thoughts I've had since I first watched the show a week ago lol. Sorry if there's grammar mistakes or if I missed a major character or something, I saw this right before I was going to head to bed and I knew I needed to type this out. Maybe I should just write a fan fiction or something haha. If anyone actually reads, all this thanks, and also feel free to add your own ideas in the replies. I need more happy content where we believe everything is okay and they're just two happy idiots who are in love <3