One interesting idea could be exploring how the babies inherit the unique ninja abilities of their parents at a very young age. For example, a baby of Naruto might show early signs of being able to control the Nine - Tails' chakra in a cute and unexpected way.
One interesting idea could be the superpowers the babies inherit. For example, a baby of Iron Man might show an early aptitude for technology or engineering. Another could be how the other Avengers interact with the babies, like Thor trying to teach them about Asgardian culture in a really fun and comical way. And also, there could be a plot where the babies are in some sort of danger and the Avengers have to use their unique skills to protect them.
One idea could be exploring the backstories of the Big Chill Babies. Maybe they have unique powers that are yet to be fully discovered. Another could be a story about their first encounter with other alien species and how they interact.
Well, one interesting idea could be seeing how the characters' magic powers might be passed down or manifest in their baby forms. For example, if Natsu's child might have some sort of fire - related magic from a very young age. Another idea could be the relationships between the different baby characters, like who would be friends and who might have little rivalries.
One idea could be that they are all babies in a magical nursery where the toys come to life and help them discover their musical talents. For example, a singing teddy bear could be their first music teacher.
You could write a fanfiction about a baby who time - travels. The baby goes to different historical periods and has an impact on the events that occur. It could be a fun and educational story as you can incorporate real historical facts and how the baby's presence changes things.
One idea could be that their babies inherit unique powers from both parents. For example, they might have Hellboy's strength and Liz's ability to control fire, but in new and unexpected ways. Maybe they use these powers to protect the world from a new, unknown threat that the regular heroes can't handle.
One interesting idea could be the combination of Naruto's ninja skills with mecha technology. For example, Naruto might use his chakra to power up the mecha in unique ways. Another idea could be the new enemies that are created specifically for this mecha - Naruto concept, like robotic shinobi that require different combat strategies compared to normal foes. Also, the design of the mecha Naruto itself can be really cool, perhaps with features inspired by the Nine - Tails.
Another idea is that Aizen somehow ends up in the Naruto universe during the time of the Fourth Great Ninja War. He could try to manipulate both sides for his own gain. But Naruto, with his ability to see the good in people, might try to reason with Aizen and show him a different path. It could be a story of redemption or a great battle if Aizen refuses.
In a 'Naruto Mermaid Fanfiction', the mermaids could be in danger from an evil organization that wants to capture them for their precious scales. Naruto teams up with a mermaid princess to stop this threat and in the process, they learn about each other's worlds.
One idea could be a crossover where Naruto and Pucca end up in the same world due to a strange ninja jutsu gone wrong. Naruto is confused by Pucca's constant chasing of Garu, and he tries to help Garu escape in his own Naruto - style way.