"Candidates for adoption must be 15 years or older, single and without a child. The emperor, empress, crown prince, crown princess, emperor emeritus and empress emeritus would not be eligible to adopt. This means only seven members along all existing branches of the imperial family, including the Mikasas and the Takamados, would be eligible to become adoptive parents.

Princess Nobuko of Mikasa, the younger sister of veteran LDP lawmaker Taro Aso, would also be eligible to adopt."

The Emperor and Empress cannot adopt under this law nor can the Crown Prince, Crown Princess.
Going back, if Naruhito and Masako had another daughter, under the present law, they could not adopt a boy and he would be Crown Prince/Successor same with Akishino and Kiko. There was never a question that Kiko's last pregnancy might be a girl, she was carrying a boy.