A really cool storyline could be a time - travel one. The Powerpuff Girls accidentally get sent to a different time period in the DC Universe where they meet a younger version of the Justice League. They then have to work together to get back to their own time while also dealing with threats that exist in this new time period. This would allow for a lot of character development and interesting interactions between the characters from both franchises.
One interesting storyline could be the Powerpuff Girls teaming up with the Justice League to fight a common, super - powerful enemy from outer space. The Powerpuff Girls' unique powers and small - but - mighty nature could add a new dynamic to the Justice League's battles.
A third interesting storyline might involve a secret alliance between a member of the Justice League and a villain that the Young Justice team discovers. This discovery then sets off a chain of events where the Young Justice team has to decide whether to trust the rest of the Justice League and how to handle this new information without causing a complete breakdown in the superhero community.
In some fanfictions, there might be a story where the Powerpuff Girls have to deal with a personal conflict among themselves. Like Blossom might start feeling left out because Bubbles and Buttercup are getting closer. This internal struggle could then affect their ability to fight crime as a team. They would have to work through their relationship issues to be able to function as the city's protectors again.
The Powerpuff Girls themselves, Blossom, Bubbles, and Buttercup, are usually very popular. Their cute yet powerful personas make them stand out. Also, Superman from the Justice League is often a fan favorite in these crossovers. His super strength and all - around good - guy image blend well with the Powerpuff Girls' world.
One interesting storyline could be Thor joining the Justice League during a major threat from a powerful cosmic entity. His godly powers, like his lightning and strength, would add a new dynamic to the team's battles. For example, he could team up with Superman to take on an enemy that can manipulate gravity. Their combined strength and different power sets would make for an epic showdown.
One interesting storyline could be Wolverine being brought into the Justice League's universe due to a multiverse rift. He might initially clash with some members like Batman over their different approaches to justice. Batman is all about strategy and preparation, while Wolverine is more of a brawler. But over time, they learn to work together and Wolverine's healing factor and combat skills prove invaluable against a powerful new threat that the Justice League alone couldn't handle.
There could be a story where an OC has the power of telepathy but is initially shunned by the Justice League because they're afraid of this new power. The OC then has to go through a journey of self - discovery and also convince the League that they can be an asset, perhaps by using their power to stop an alien invasion that was hidden from the League's normal detection methods.
In some fanfictions, there could be a plot where the powers of the Powerpuff Girls Z combine in a new way when they are in the RWBY world. For example, their chemical X - like powers could interact with the Dust in RWBY to create unique abilities. This could lead to them training the RWBY characters in this new form of power usage.
One interesting storyline could be an OC (Original Character) who has a unique power that the Powerpuff Girls Z initially mistake for a threat. For example, an OC with the power to control plants might start making the city overgrown with beautiful flowers and trees. The Powerpuff Girls Z think it's chaos at first, but then realize the OC just wants to make the city more beautiful. Another could be an OC from another dimension who comes to their world and has to learn how to fit in while also helping the Powerpuff Girls Z fight a new villain. There might also be a story where an OC has a power that complements the abilities of the Powerpuff Girls Z, like an OC who can manipulate time a little bit and helps them prevent disasters before they happen.
Another storyline might involve Danny being wrongly accused by the Justice League of a crime he didn't commit. As he tries to prove his innocence, he uncovers a deeper conspiracy that threatens both the human world and the Ghost Zone. He has to use his wits and powers to clear his name and save the day.
Perhaps a story where Hermione is seen as a threat by some members of the Justice League at first. They don't understand her magic and think it could be too dangerous. But then, when a common enemy emerges that is immune to their usual methods of attack, Hermione steps in and uses her magic to save the day. This could lead to a story of acceptance and friendship, with her joining the League on certain missions and using her unique skills to complement the League's existing powers.