Common themes are transformation, not just in the sense of the Miraculous transformations but also in the characters' personalities. Hawk Moth might transform from a pure - evil figure to something more complex. Another is the exploration of the Miraculous powers. Fanfics often dig deeper into how the powers work and how they can be used in different ways against Hawk Moth. And secrecy is a big theme. Keeping identities secret, Hawk Moth's secrets, and how those secrets impact the story.
One great fanfiction is 'The Hawk Moth's Redemption'. It explores an alternate universe where Hawk Moth starts to question his motives. Another interesting one is 'Ladybug and Hawk Moth's Unlikely Alliance' which has a unique plot of the two having to team up for a common cause. And 'Hawk Moth's Hidden Past' delves into his backstory and how he became the villain we know.
There could be fanfictions where Gabriel as Hawk Moth teams up with unlikely allies. Perhaps he realizes that his way of doing things is wrong and joins forces with Ladybug and Chat Noir to fight a greater evil that threatens the entire city or even the world.
It could be Gabriel Agreste. He has been a suspect in the original series for various reasons. He is often absent and has a lot of secrets. So in the fanfiction, it would make sense if he was finally revealed as Hawk Moth.
I'm not sure of specific titles off the top of my head, but you can check out fanfiction websites like Archive of Our Own. There are likely many stories there that focus on Gabriel as Hawk Moth. You can look for ones with high ratings and lots of reviews.
Well, in this fanfiction's final battle, there could be new Miraculous holders joining Ladybug and Cat Noir. They could bring in fresh powers and strategies. Also, the final battle might take place in a completely unexpected location, like inside a digital realm created by Hawkmoth's new power. Ladybug and Cat Noir would have to adapt quickly to this strange environment to win.
In the 'the final battle miraculous ladybug fanfiction', perhaps there is a twist where Hawkmoth teams up with an unexpected villain from the past. This forces the Miraculous heroes to not only fight a stronger enemy but also deal with the complex dynamic of two villains working together. Ladybug has to come up with a new strategy that involves all the different Miraculous powers at their disposal.
Ladybug would likely be the most important. She is the main hero and has the power of creation. In the final battle, her ability to think on her feet and come up with creative solutions using her Lucky Charm will be crucial. She also has the leadership skills to rally the other Miraculous holders.