This is my suggestion for a Barbie Justice League, but then I got thinking, what kind of threat would require them to take Action in the first place? Then it hit me. What better threat to Barbie, than another Barbie?
A Barbie who *sees*. A girl that's aware of the Barbie multiverse, plagued by visions of their lives. It would start out small as a kid, people thinking she just had an active imagination, but it would get worse as she grew, getting closer to the other's adventures. A normal girl seeing the extraordinary. She wasn't a superhero, princess, a spy, a musketeer, a mermaid, an alien, a magic user or anything else. She didn't have friends, or boyfriends, or girlfriends, or even siblings. No one would want to hang around the crazy girl who sees things that aren't there. It would become too much. Too much noise, to many lives going through her head at once, she'd just want quiet. So she'd decide to make it stop. With her knowledge of the other worlds, skills, technology, magics, abilities, she'd make it her mission to eradicate the very concept of Barbie, destroying them all in one strike. It didn't matter if without Barbie every dimension she's at the center of would end. She would be a threat no single Barbie could stop. So how would she handle a team of them? Or better yet, how would they handle her?
Based a lot on Owlman from the movie, the episode And Then There Were None from Ben 10 Omniverse, and the many times Shredder tried this against the Turtles. I think it's cool.