Another variation: The current youngest (and at this time third in line) heir in the Japanese Imperial line is the 6-year old Prince Hisahito, nephew to the current Crown Prince. The young Prince is also the only male heir eligible under the current laws of succession, after his father and uncle. Considering the recent news of the pregnancy of Kate, Duchess of Cambridge and wife of William, second in line to the throne, under the English Law of Succession her first child, boy OR girl, would then also be in the direct line. If Kate's child is a girl, she would be an appropriate age for the young Prince, should he be interested.
The twist: what of a marriage between these two future monarchs, the future Emperor and the future Queen, joining the Chrysanthemum Throne with the Lion and Unicorn?