The thing about Koby vs Kuzan is that it just doesn't make sense thematically as Koby's final opponent/obstacle. Koby's pretty much been defined as a hero since the timeskip. Koby's monumental victory should be over someone who represents some significant corrupt/evil force/movement – that's how a heroic victory is traditionally narrativized and achieved. You need people all over the world cheering on as Koby fights a capital B Badguy. You simply won't get this kind of scenario with Kuzan. Koby fighting Aramaki is several times more likely than Kuzan, if only because Kuzan's role in the story is that of anti-hero. But even Aramaki isn't that likely of a candidate imo.
Look carefully at the people vehemently pushing for Koby to take down Kuzan. They don't have a solid reason, they just REALLY dislike Kuzan like Tac or Ha001 for instance and hope for him to have the most embarrassing end possible.