In a 'reborn as superman fanfiction', you could explore the idea of Superman being reborn into a future where Earth has become a desolate place due to environmental disasters. He has to not only use his powers to protect the remaining humans but also find a way to restore the planet. This would add an interesting ecological aspect to the story, as Superman tries to use his powers like super - speed to plant trees all over the world and his freeze breath to create ice caps again.
One idea could be creating an OC (Original Character) who is a young journalist from a different planet. They come to Earth and end up working with Clark Kent at the Daily Planet. Their different perspective on Earth and Superman's world can create a lot of interesting storylines.
A superman crossover with Wonder Woman would also be great. Maybe in a story where they have to travel to another dimension to stop an ancient evil. Their different powers and personalities would create a lot of interesting interactions, like Superman's more down - to - earth (even though he's from another planet) approach compared to Wonder Woman's Amazonian heritage.
One idea could be a battle between Goku and Superman to determine who is the strongest in the universe. They might start off as rivals but then realize they have more in common than they thought and team up against a greater threat like Darkseid and Frieza combined.
One idea could be that Superman arrives in the RWBY world and has to learn about Dust and its powers. He could team up with Team RWBY to fight a common enemy, like a powerful Grimm that is immune to normal attacks but not to Superman's strength. Another idea is that Ruby discovers she has some Kryptonian-like abilities and Superman helps her understand and control them.
One idea could be that the OC is a scientist who discovers a new form of Kryptonite and Superman has to work with the OC to keep it out of the wrong hands. The OC could also have a unique power that compliments Superman's, like the ability to control technology which helps Superman in battles against tech - based villains.
One idea could be the two heroes having to hide their relationship from the public while still saving the world together. They might face challenges like dealing with their different approaches to justice within the context of their romance.
Maybe there's a plot where a powerful alien force arrives on Earth and starts causing chaos. Superman is having a hard time dealing with it alone as the aliens have some new form of kryptonite - like energy. Catwoman, with her stealth and street - smart skills, discovers a way to neutralize the kryptonite - like effect, and they join forces to save the day.
One idea could be a story where Batman and Superman's kids team up to solve a mystery in Gotham City. Maybe they find a strange device that's causing chaos and they have to use their combined skills, like Batman's detective skills and Superman's super strength, to figure it out.
One idea could be a story where Batman and Superman have to team up to stop a new villain that has the power to manipulate time. This would force them to put aside their differences and really work together in a way they haven't before.
One idea could be that an alternate - universe version of Superman comes to their world. This could lead to a power struggle as both Supermen have different ideas on how to protect the city. Another plot could involve Lois getting superpowers temporarily and having to learn how to use them while also dealing with her regular reporting job.