The Imperial household is having to revise the law to prevent a constitutional crisis as no male has been born into the family since 1965.
The present law, only instituted in 1947, restricts the line of succession to the oldest male.
Revising the law to allow the oldest child, regardless of gender, allows the popular Princess Aiko, who is 4, to ascend to the thrown when the time comes.
Unfortunately, the conservative element of the Diet that imposed the original law is attempting to scuttle this attempt at liberalising the Imperial house, even though the majority of the public support Princess Aiko becoming next in line.
I am hoping that the people of Japan, and Princess Aiko, win and the law is reformed to allow female successions.
As to what to call a commoner marrying into the imperial family, another concern, they should perhaps consider a British-like suffix of consort.
So if Princess Aiko married a commoner then his title would be Empress-consort.