The Unstable Miraculous (LadyNoir)

When Marinette finally gains the courage to tell Adrien she likes him, he turns her down. His heart belongs to another. Heartbroken, Marinette goes on patrol as Ladybug with Chat Noir that day, and gets akumatized.
Now the miraculouses are thrown out of balance. The Earrings of Creation, meant to purify, are now the akumatized object. Because the miraculouses are unstable, The Unstable Miraculous, The Bluebird Miraculous, rises to create balance again. It wasn't made right, and is unpredictable, meant to only be used as a last resort.
One of The Bluebird's Miraculous' quirks is it can only be wielded by a member of a certain bloodline. Luckily, Marinette's cousin Marilyn from China has been staying at her house for some time, and is the only one in Paris able to wield the miraculous.
Once Ladybug is deakumatized, she realizes that maybe she doesn't need Adrien, she already has someone who loves her, maybe it's time she started flirting back with the kitty.
Meanwhile, Marilyn realises that another one of the Bluebird Miraculous' quirks is that Marilyn doesn't only have powers when she's transformed. And the more she uses them, the more unpredictable the other miraculous become. She's either the key to defeating Hawkmoth...
Or accidentally revealing Ladybug and Chat Noir's identities to the whole world.

I do not own Miraculous ladybug.
Some characters, like Marilyn, are my own. I also own the plot.