The US built a number of "pumpkin" bombs, which were the same dimensions and weight as the Fat Man atomic bomb, but filled with high explosive. 49 were dropped on Japan by the 509th Group (the Silverplate B-29s) as practice for the atomic-bomb missions. One pilot (without authorization) tried to drop a "pumpkin" on the Imperial Palace in Tokyo.
(My "source" for this is a vaguely remembered Reader's Digest article from at least 40 years ago; it was titled "<so-and-so>'s Pumpkin Raid on Hirohito".)
The bomb missed. WI if it hit? A "pumpkin" contained 2.9 tonnes of HE; it would do considerable damage, and quite possibly kill Hirohito.
In which case, it probably becomes impossible for Japan to surrender. The hard-liners in the Japanese high command would demand revenge for the death of the Emperor, This might include the murder of all Allied PoWs, the murder of all interned Allied civilians, or even a general massacre of anyone in Japanese-occupied territory. Japanese reprisals against Chinese civilians for the Doolittle raid (because the raiders crash-landed there and escaped overland) approached 250,000 deaths. The death of the sacred Emperor would call for slaughter many times greater (IMO).
After that, the Allied war against Japan would become a war of extermination. The surviving Japanese, if any, would not like Americans much.