An idea for a future legendary could be when the MC just begins looking for the legendary they come to him wanting to be breed by him since the have been following and info on him and be a super fan of his so don't want to wait any longer. But either a group (criminal group?) kidnaps the legendary to breed then take over the world or by a group of super fans of the legendary want her for themselves and think they deserve her more than someone who does not know much about her. Two possible legendary that could be used is Enamorus or Meloetta. He would then have to find them then deal with who ever took them through battle.
Enamorus does respect and brings new life into the world so the MC getting legends pregnant would excite her to find someone to fulfill her wish of making her a mother instead of just bringing new life to others. The criminal gang could be used for her and the super fans for Meloetta.
Melloetta could have the personality of not wanting to be treated as an untouchable object being an idol and wants to go with the MC as soon as possible but super fans get in the way. Both legends don't do their best at resisting the capture since they don't want to show a bad side to the MC since they REALLY want him to love them but learn he will love them regardless of how they act and that makes them love him even more especially that they want to have massive pregnant belly's and be inflated by cum often and don't need to act like a perfect idol anymore.