Maybe Naruto gets tired of always chasing after Sasuke. He has his own life and dreams too. He might think that he has sacrificed enough of his time and energy on Sasuke, and it's time to focus on himself and the village. He could start to believe that Sasuke has made his own choices and he can't keep forcing Sasuke to come back.
The dynamic between Naruto and Sasuke would be completely different. Instead of the usual pursuit, there would be a sense of finality. Naruto might start to avoid Sasuke, which would change the way Sasuke views Naruto too.
One reason could be exhaustion. Naruto has been through a lot in the original story. In a fanfic, if he is continuously put through more and more difficult situations without proper breaks, he might just give up out of sheer tiredness.
If Naruto gives up on Sasuke in fanfiction, the storyline could take a very different turn. For one, the whole dynamic of their relationship, which has been a central part of the original Naruto story, would be completely disrupted. Naruto has always been determined to bring Sasuke back to the village. Without that drive from Naruto, Sasuke might become even more isolated and perhaps fall deeper into the darkness. This could also lead to different power alignments in the ninja world. Allies of Naruto might question his change of heart, and enemies could take advantage of the rift between the two former friends.
The story would take a very different turn. The dynamic between Naruto and the rest of the characters would change. For example, Naruto might become more focused on protecting the village without the distraction of Sasuke. His relationships with other friends like Sakura might also be affected. Sakura might be disappointed in Naruto at first for giving up, but then they could bond over the loss of Sasuke in a different way.
Well, if Naruto were to give birth to Sasuke's baby in a fanfic, it could be a story full of drama. The ninja world would be in shock, and their friends like Sakura and Kakashi would have very mixed reactions. There would be challenges in raising the child, especially considering the complex relationship between Naruto and Sasuke. Maybe the story could focus on how they overcome the prejudice and difficulties in this new situation and create a new family dynamic that is different from the norm in the Naruto world.
One reason could be the sacrifices he has to make for the village. Naruto is so dedicated to protecting Konoha that he might feel he doesn't have time for love. His responsibilities as Hokage are immense, and he has to constantly be on guard against threats. Another reason could be past traumas. He has lost many people he cared about, like Jiraiya. This could make him afraid of getting close to someone again and losing them, so he gives up on love.
If Naruto gives up his dream, Sasuke might view him as a weakling. Sasuke has always been competitive with Naruto, and Naruto's giving up could make Sasuke lose respect for him. Their relationship could become strained as Sasuke may distance himself from Naruto, thinking he is no longer the worthy rival he once was.
One of the main challenges would be the social acceptance. The ninja world has certain norms and values, and this situation would go against them. Their friends and comrades might have a hard time understanding or accepting it. Another challenge could be the safety of the baby. There could be those who see this as an abomination and try to harm the child. Also, Naruto and Sasuke themselves might struggle with the new responsibilities of parenthood, especially given their own complex pasts and the unique circumstances of the birth.