The baby could inherit Naruto's chakra reserves and his ability to use the Rasengan. Since Naruto has an enormous amount of chakra, it would make sense for the baby to also have a large chakra pool. The Rasengan is one of Naruto's signature moves, and it could be passed down genetically or through training that Naruto would give the baby as it grows up.
Perhaps it could be a story of friendship and sacrifice. Tenten finds an orphan baby during a mission. Naruto, being the hero who always wants to protect the weak, offers to help her. They decide to raise the baby together in a platonic way. Naruto uses his experience of growing up alone to make sure the baby feels loved, while Tenten uses her skills to provide for the baby's physical needs like food and shelter.
Tenten would likely be deeply traumatized. Naruto was not only a comrade but also a symbol of hope and strength. His death in protecting her would make her feel guilty. She might start to question her own worthiness of being saved at the cost of Naruto's life.
Their friends would likely be very excited. Naruto would probably be one of the first to visit and offer to babysit. He might bring some of his favorite ramen for the new parents. Sakura would be there to offer medical advice, being a great medic - ninja. And Shikamaru might complain about how much work having a baby is, but still be happy for them.
Well, the story could be about how Neji and Tenten's relationship evolves as they become parents. Maybe it starts with Tenten finding out she's pregnant. Neji would be a very protective father, using his Byakugan skills to keep an eye on the baby even before it's born. And Tenten would be teaching the baby all about her weapon - using skills when it grows up.
They would have to develop a strong bond as a couple first. Since they are not a canon couple, the fanfic could show how they gradually fall in love. Maybe they start off as friends, and through shared missions and experiences, they realize their feelings for each other. Once they have the baby, their relationship would become more about family. They would support each other in raising the child, and their love would grow deeper as they see their baby grow.
Naruto might be really confused at first. He's used to people being friendly or having normal crushes, but a yandere is a whole different level. He'd probably try to talk to her and understand why she's acting so strangely.
Their relationship could start with mutual respect. Naruto respects Tenten's dedication to her weapon - based ninjutsu, and Tenten respects Naruto's strength and determination. They might start training together more often, which would lead to them spending a lot of time together. This time together would allow them to open up to each other about their pasts, their fears, and their hopes for the future. As they grow closer, they would start to develop feelings for each other, and these feelings would eventually lead to their marriage.
Maybe Rock Lee could be an important character. He could come to the orphanage to help with the physical training of the orphans. He would be a great inspiration for them with his hard - working attitude. Also, Neji could be involved. He could use his Byakugan to keep an eye on the orphanage and make sure there are no threats approaching.
In this fanfiction scenario, Naruto giving Tenten a sword could be a really interesting plot point. Maybe Naruto found a special sword during one of his adventures and thought Tenten, being a weapons expert, would appreciate it. It could lead to Tenten developing new fighting techniques with the sword, and perhaps she would use it in future battles against powerful enemies.