I will be honest, I can't see Koichi or any other Vigilantes characters coming into the main story in any significant role. Hori has never even hinted at them.
If international heroes do appear, it will probably be ones we have already seen in the main story before: Salaam, Big Red Dot, Electoplant, Nyikang, and Pankration.
Even Captain Celebrity is a stretch since he also hasn't been mentioned in the main story. Hori would have to do a completely new introduction for both Koichi and CC if he wants them to be anything more than glorified cameos.
But, yeah, if Koichi does miraculously appear, it will probably be in the fight against Prime AFO.
Like imagine a team of Endeavor, Koichi, Captain Celebrity, and maybe a few others against AFO. I think they would hold out well even without Deku. (We could also get a Koichi-Hawks speed comparison). O'clock was kinda the High-speed hero of his generation & Hawks is this generation's speed hero. And Koichi is basically O'clock's successor and faster than his master.