Feud of the Miracle Boxes
What if the mage who made the Miraculous also crafted the two Miracle Boxes? What if he passed down the boxes to his students/children and it ignited a feud? What if Lila and Marinette are the latest additions to that feud?
Basically, after creating the Miraculous and Miracle Boxes, the mage gave them to his children/students to protect. However, the mage chose to give the Miracle Box with the two most important Miraculous (Ladybug and Black Cat) to the more responsible one, worried what the other might do with that power.
This decision and clear favoritism anger the inheritor of the other box. Driven by envy and a lust for the power that comes with the two Miraculous, the second inheritor used their Miracle Box to try to steal the Chinese one. However, the first inheritor prevents this and escapes, causing the second heir and their allies to begin hunting them.
This hunt has lasted for generations. The second heir's successors do their best to accomplish the person's goal. Eventually, this leads to a battle between the two newest inheritors, Marinette and Lila.
Lila moves to Paris early and manages to sense Marinette's Miracle Box in Paris. This causes her to break into her Miracle Box for a Miraculous to cause a commotion that will force Marinette into action (possibly unknowingly). Maybe her predecessors managed to gain the Butterfly Miraculous and she either uses it or has someone use it for her. Or she could have a completely different Miraculous.
Maybe Lila uses people like Chloe and Gabriel as pawns to do her dirty work for her. Meanwhile, Marinette might not have found out about her new destiny until Lila makes a move, causing her to have to adapt quickly to her new job.
To add something else. Maybe both Marinette and Lila have the natural ability to use magic. So, they can use powers other than the Miraculous to fight.