The Taishō Emperor (Yoshihito) was historically the eldest child of the Meiji Emperor (Mutsuhito) who lived to adulthood, and also the only male child to live to adulthood. At the young age of three weeks, he contracted cerebral meningitis. What if he'd died of it?
If we assume that Mutsuhito has no further male issue that live to adulthood, it seems likely that Japan would allow a female Empress (Princess Masako, assuming limited butterflies/morphic twins) to ascend to the throne upon Mutsuhito's death (IIRC, they nearly did when Hirohito failed to produce a male heir). How would this affect Japan, especially in relation to democracy, the military and feminism?
Note: I wasn't sure whether to put this in the After 1900 or Before 1900, so I went with After, since most of its effects wouldn't be seen until the Meiji Emperor dies, which would presumably be post-1900. If it should be moved, feel free.