A weaker/nonexistent Japanese Empire would definitely be a start. It would be interesting to see how Korea develops as a still-united independent kingdom in the 20th century, without Japanese colonial interference or Cold War division.
It'd be really interesting, because Korea's latent potential as a power is surprisingly potent when you think about it. The two Koreas IOTL already have a higher combined population than the UK or France, and you could easily see it be even higher in a TL where there's no psychotic dictatorship or crippling famine in the north. The peninsula also has more mineral wealth than Japan, as well. So long as it doesn't suffer really catastrophic leadership, you would almost certainly expect this Korea to be in the top 10 global economies by the present day (South Korea is number 12 right now), and probably somewhere around number 20 for global population, although that's more up in the air with butterflies in other places. They may or may not get nuclear weapons, but that depends on a ton of other factors, and obviously plenty of significant powers don't have them right now, like Germany, Japan, Turkey, Brazil, OTL South Korea, so who knows.
I wonder about their per capita GDP, since South Korea is still a bit below where countries like France or Germany or especially the US are. Still, it has gotten past the middle income trap where countries like Brazil or Russia or South Africa have stalled out, so that's definitely another point in their favor. They're by far the largest of the East Asian Tigers, too.