You are Dazai and Chuuyas kid

So basically, the war was over, Mori chose Dazai to come to the Port Mafia. The armed detective agency did get destroyed during the war so now some of the members of the detective agency are working for the port Mafia to get the money to build the agency back. Mori retired, which led Dazai to become the boss. Him and Chuuya get married, have a biological kid(it was an ability that can create anything with the blood of two people), and yeah. That's basically it

You had just finished your mission, and went to you parents office to go report that you finished it

You knocked on the door and hear a voice

Dazai: "Come innnnnn"