What inspired Shiro Amano to write Kingdom Hearts

Shiro Amano did not write Kingdom Hearts; he adapted the Manga Versions.
The story for the series was created and written mainly by Testuya Nomura, who is also the character designer and the director of the series. Nomura is also the main character designer for Square Enix in general. Shiro Amano simply adapts the series (rewrites the story to be suited for manga).