My father used to joke that Hitler was the one who did the most to liberate Korea because, without WWII, the USA and USSR didn't have that much interest nor reason to cut up the Japanese Empire (Koreans had tried getting independence at Versailles in accordance with the self-determination of the 14 Points but no one there actually cared about anything east of the Urals so that amounted to nil) and Korea wasn't getting independence on its own, not when Japan outnumbered and outgunned the peninsula. A less expansionistic Japan would've kept Korea for a couple more decades, at the very least. Then it comes down to Japan's colonial policies and whether or not Japan treats Korea as a buffer state/resource colony/headway into China.
Taiwan currently is quite fond of Japan, interestingly enough, as it was a model colony which was treated quite nicely in contrast to most other cases of colonial rule, apparently.
Invading Manchuria gets Japan on bad terms with most of the other Great Powers and makes for a rather long border with the USSR. But it does have a large resource base, which would help.
So that ends up with a Japan with a population perhaps 400 million by the present day (Japan with 120 million, both Koreas with 75 million (South with 50, North with 25), Taiwan with 25 million, Manchuria with 110 million) that avoids the ravages of WWII, the Korean War, Chinese Civil War. Seeing as Japan was behind only the USA in terms of nominal GDP for decades (surpassed in 2010 by China) and this would strip China of Manchuria and add onto Japan the economies of Korea and Taiwan (South Korea being 11th without North Korea's mineral resources and Taiwan being 22nd), Japan would still be top dog in Asia right now.