Maybe a crossover with the Justice League. The DC Superhero Girls look up to the League, and in this fanfiction, they could be called upon to help when the League is incapacitated by a powerful magic spell. It would be a chance for the girls to prove themselves on a bigger stage.
How about a crossover with the Gotham City Sirens? Harley Quinn, Poison Ivy, and Catwoman from that group could interact with the DC Superhero Girls. There could be a lot of fun and chaos as the girls try to either reform or fight against the Sirens. Maybe they end up having to work together when a bigger threat to Gotham emerges that affects both the good and the bad guys.
A cool plot could involve Wonder Woman, Flash, and Aquaman. They find themselves in an alternate dimension where the rules of their powers are all mixed up. Flash might not be as fast, Aquaman can't communicate with sea creatures as well, and Wonder Woman's lasso doesn't work as expected. They have to figure out how to adapt and work together to get back to their own dimension while also facing the threats in this new world.
Another plot could involve Gohan being mistaken for a new metahuman in the DC universe. As he tries to find his way back home, he gets caught up in a conflict between the Justice League and a group of supervillains. His Saiyan powers could be the key to resolving the situation.
One idea could be Goku ending up in the DC universe and having a friendly spar with Superman. Their different power sources and fighting styles would make for an exciting encounter. Maybe Goku learns about Kryptonite and how it affects Superman, which gives him a new perspective on weaknesses.
One idea could be Wolverine getting transported to the DC universe and teaming up with Batman. They could combine their detective skills to solve a mystery that involves both mutants and metahumans. For example, a new villain is creating chaos in Gotham by using some sort of technology that affects Wolverine's healing factor in strange ways.
One idea could be the Boys teaming up with a DC hero like Superman, but then realizing his 'good guy' image might be a facade too. Another could be Homelander facing off against Batman in a battle of wits and power.
One idea could be that Naruto and his friends are transported to the DC Universe. Naruto's chakra could be seen as a form of magic in the DC world. He might team up with Superman to stop a powerful villain who is after both kryptonite and Naruto's chakra. Another plot could involve Batman being intrigued by Naruto's ninja skills and trying to incorporate some of them into his own combat style.
A cool plot for dc super heroes crossover fanfiction could involve Wonder Woman and Aquaman. Maybe there's a threat that endangers both Themyscira and Atlantis. They must combine their unique skills, like Wonder Woman's combat abilities and Aquaman's control over the sea, to save their respective homes. Also, a Martian Manhunter and Cyborg crossover could be interesting. The Martian Manhunter's shape - shifting and telepathy, along with Cyborg's technological prowess, could be used to stop a cyber - psychic threat.
One idea could be that Naruto, with his Mokuton abilities, gets transported to the DC universe. He might team up with Green Arrow in Star City. Since Naruto can create and control plants, he could help Green Arrow by using the Mokuton to create natural barriers during battles or to track enemies by sensing their movements through the plants. Another plot could involve Naruto meeting Poison Ivy. They could have an interesting dynamic as both have a connection to plants. Naruto might try to convince her to use her powers for good instead of causing chaos.
One interesting plot idea could be that a character from DC, like Batman, somehow ends up in the Watchmen universe and has to deal with the moral ambiguity there. He might find himself clashing with Rorschach over their different views on justice.