Most probably the case if Japan did not surrender peacefully. If it had, the US could have made better terms for Japan, and the peninsula could have well been at least an autonomous province under the Japanese government.
I think that would be foolish on both the US and Japan's parts. Japanese imperialism would've been stopped once and for all. I think it would be silly to say "okay guys don't rampage over the Pacific ever again but you can keep Korea". Better terms yes, but I think empire would be a non-starter. Besides, Japanese rule of Korea was repressive. Not sure if a Japanese government would be willing to deal with the costs of keeping territory with such a rebellious populous.