my challenge is to make korea as powerful a possible in this timeline. For example: no Japanese colonization, no division. just try not to be too ASB
No division...
zeppelinair has a pretty good timeline on this.
IMO I think a temporary ROC victory in the Chinese Civil War (you can make it permanent if you want) would mean that China begins to assert itself in Korea. Normally, this isn't exactly a
good thing for Korea... but the Korean War isn't normal.
The Chinese government supported the Provisional Government of the Republic of Korea, and Chiang could probably get the US to agree. The only remaining challenge is the Soviets, and a Chinese victory means that they can't control the Norks as well because Manchuria is blocked off to them.
https://en.wikipedia.org/wiki/Provisional_Government_of_the_Republic_of_Korea
This itself means no initial division, so Korea is free for wanks. You can destroy the ROC later if you want, but this initial step is important for a unified Korea.
No Japanese colonization...
I'd say another China-wank would probably help here (but not too much). Or a Russia wank.
Basically, you can get the Russians and the Japanese to fight to a standstill where both sides are too exhausted to do anything. Or the Chinese and Japanese. Or the Russians and the Chinese and the Japanese (though the Chinese and Russians would be on the same side).
But if you want an earlier Korea wank, no Imjin war would definitely help. I wrote a
timeline about this (haven't finished yet, though, and it's a different type of Korea-wank).
https://www.alternatehistory.com/discussion/showthread.php?t=374116
In my TL, the Manchus overrun Korea as IOTL - but unlike IOTL, they don't succeed in invading China. This means that they're trapped in Korea. I haven't finished this TL, but from here, it can go two ways.
1. The Qing dynasty in Korea (called Balhae) is under threat from the north/west because of the Ming Chinese, who don't exactly like them, and from the south/east because of the Japanese, who want to conquer them. In other words, they're trapped between two hostile powers, and unlike the two 'behemoths' surrounding them, they have to change if they want to survive. They look west for a solution.
2. The Qing dynasty in Korea (called Balhae) is doomed to failure. The native Korean populace hates them with unrivaled fiery passion, but the Qing have brought in strong horseback armies. The native Koreans must find a way to combat the stagnant Qing - and what better way than looking west?
Obviously, having the Wanli Emperor not defend Korea is a very unique POD that most wouldn't choose. Consider a few other Imjin War PODs:
1. Wanli intervenes from the start, sending in a huger load of troops. The Japanese are crushed, and Korea is not as war-torn. As a result, they're better able to defend against the Manchus and actually help the Ming out. By helping the Ming out, Liaodong is safe, as is Korea. Korea now has the chance to expand north into Manchu territory.
2. I forgot what I was going to write here.
Point is, there's a bunch of PODs. I encourage you to try and write our own TL (I promise it's really fun)!