One idea could be a crossover where Naruto characters end up in the world of Nisekoi. For example, Naruto could meet Raku and they team up for some sort of mission in the Nisekoi city. It would be really cool to see Naruto's ninja skills in that setting.
Chitoge could also be a great focus. Her tough - exterior - but - soft - inside personality would be really fun to explore in the Naruto universe. Maybe she could learn some of Naruto's values like never giving up and it could change her perspective on her own relationships.
One common theme is the idea of unexpected change. The pregnancy is an unplanned event that shakes up the characters' lives. Another theme could be love and support. Raku and Chitoge might have to show their love for each other in new ways as they face this big change. Family also often plays a role, with their families having different reactions to the pregnancy.
Nisekoi After Story would most likely build on the original plot of Nisekoi. The main characters who were in a fake relationship in the original might face new challenges that test their real feelings for each other. There could be more exploration of their backstories, their families' influence on their relationships, and how they grow as individuals while dealing with their complex love situation. It might also show how their friendships with other characters evolve as a result of the relationship drama.
It's about a love story filled with misunderstandings and complex relationships. The main character gets involved in a fake relationship which gradually evolves into something more complicated and emotional.