Here are two ideas up for adoption.
Baa-chan
Tsunade and Jiriya had a one night stand.The result is Arashi..though Jiriya is unaware he is his son.Things go as per manga till after Tsunade becomes Hokage. She is going through Narutos records when she screamns in rage and frustration.
Sarutobi never told her Arashis child had survived nor that he was the Kyuubi container.....
Clock.
Mitaroshi Ankos biological clock has just gone off......and she chooses Naruto to be the father of her baby after thinking through all of the potential canidates.....
Baa-chan
Tsunade and Jiriya had a one night stand.The result is Arashi..though Jiriya is unaware he is his son.Things go as per manga till after Tsunade becomes Hokage. She is going through Narutos records when she screamns in rage and frustration.
Sarutobi never told her Arashis child had survived nor that he was the Kyuubi container.....
Clock.
Mitaroshi Ankos biological clock has just gone off......and she chooses Naruto to be the father of her baby after thinking through all of the potential canidates.....