How are you going to do Lila? Will you make her into an interesting character, or will she be Roger Smith from American Dad?
I've got a couple of fix pitches for Lila under the "Lila Deserves Better" and "Lila Salt" tags, but my personal preference is to just not use Lila at all. I cannot stand her and the way she poisons everyone around her, making them all look terrible. To make Lila work, you need to fundamentally change her writing and the way everyone else interacts with her, basically make her an OC, and I just don't have any interest in doing that when I don't need her to make the story work.
Lila is the kind of character I'd only introduce if I was forced to setup a sequel or prolong the series past the Gabriel arc. If I had full creative control, then I wouldn't do that. The story would end with Gabriel's defeat because it's incredibly hard to top Gabriel as a villain. His motivation and ties to Adrien make him infinitely more interesting than any other villain could ever hope to be. A villain being bigger and badder just can't compete with the emotional gut punch of the villain being so closely tied to the heroes. No matter what powers you give them, they will always be a downgrade.
Even if they give Lila some wacky tie to the heroes, it's going to feel far less genuine and compelling. I'm fully expecting that tie to not exist or for it to be something bonkers like Lila being from the future. Either way, I'm not excited.
I could obviously use Lila as a petty mean girl, but I have Chloe for that. Even if I redeemed Chloe, I probably wouldn't bring Lila in to replace her because Chloe's redemption would be pretty drawn out. By the time it's ending, we'd be nearing Gabriel's defeat and it would be odd to bring Lila in then. The closer we get to the end, the less the civilian side should matter. You should be kicking off reveals and completely ruining any hope of petty school drama feeling worth discussing.
And if I couldn't draw out Chloe's redemption due to the show's format? Then I wouldn't redeem her because her redemption requires the type of format that lets you draw it out.
As a final note, I don't dislike the idea of the Miraculous team fighting someone other than Gabriel. I think you'd just need to do one of three things to make it work:
Have those fights come pre-Gabriel. Let the first few seasons be normal superhero nonsense, but the existence of the miraculous gets Gabriel searching for a way to save Emilie, leading him to find the butterfly. Maybe Lila starts with the butterfly and Gabriel gets it? Either way, he now becomes the final boss, so to speak
Let Adrien be a villain during the Gabriel arc and have the rest of the show be his redemption as the heroes fight a new threat. That maintains the emotional core that canon has lost while separating that core from the villain side of things.
Add a massive timeskip and have the show be about the heroes as older teens or adults. Removing us from Gabriel like that lets his loss feel less impactful while adding a new element of the audience wanting to see how everyone has changed over time. Still a lesser emotional core, but at least one exists.