You could write a fanfiction where Aspheera accidentally releases an even more dangerous force while trying to regain her full power. The ninjas then have to work with her, despite their differences, to stop this new threat. This would involve a lot of character development for both the ninjas and Aspheera as they learn to trust each other.
Zane would also be a good choice. Zane's logical thinking could contrast with Aspheera's more emotional and power - hungry nature. Maybe he could try to reason with her and understand her motives better than the others.
One idea could be exploring the backstories of the characters from the Hands of Time arc in more detail. For example, the relationship between Kai and Nya's parents before they disappeared. Another could be a time - travel adventure where the ninja go back further in time than in the original story to prevent an even greater threat.
A possible plot for this crossover fanfiction is that the worlds of Ninjago and Chima start to merge due to some magical mishap. This causes chaos as the different species and cultures collide. The heroes then have to figure out how to separate the worlds again. For example, the ninja from Ninjago might have to work with the lions from Chima. Another idea is that a group of villains from both worlds team up and the heroes have to form an alliance of their own. They would need to overcome their differences and learn from each other's fighting styles. There could also be a story where a Ninjago character is transported to Chima and discovers a long - lost relative from there, which adds a personal touch to the adventure.
One idea could be the next generation facing a new, more powerful enemy that combines the powers of all the previous villains. They'd have to learn from their parents' stories and develop new skills to defeat it.
One idea could be a time - travel plot where Lloyd and Nya are sent back in time to the early days of Ninjago. They have to work together to find a way back while also ensuring that they don't accidentally change the future. Another plot could involve them getting trapped in a mysterious realm and having to rely on each other's unique skills to escape. For example, Lloyd's elemental powers and Nya's water - based abilities.
One idea could be that Naruto and the Ninjago characters get transported to a shared dimension. They have to learn each other's fighting styles to defeat a common enemy. For example, Naruto could teach the Ninjago ninjas about chakra while they teach him about Spinjitzu.
One idea could be Kai getting infected by the dark matter and his struggle to regain his true self. He might have to face his inner demons while his friends try to find a cure.
One idea could be exploring the sister's backstory in more detail. Maybe she had a special training or a hidden power that was only known to a few. Another could be a story where she gets separated from the main group and has to find her way back while facing her own personal demons.
A plot idea could be that there's a prophecy about their unborn child having some extraordinary power. This would put them in danger as various villains in Ninjago would want to either capture or stop the birth of the baby. Zane and Pixal would then have to fight to protect their family.
One idea could be the ninjas traveling back in time to the very beginning of Ninjago's creation. They could witness the forging of the Golden Weapons and try to prevent an evil force from stealing them at that time. This would change the whole course of Ninjago's history and create a whole new set of challenges for the ninjas.