Some last-minute Nemesis theories
The main plot of season 6 of Miraculous has been about Marinette trying to maintain her lie about Monarch's true identity. It's only natural that in the finale it all comes crashing down. With this in mind, let's think about how this can happen. SPOILERS are present through Secret Protocol!
First, for an episode like this, it's important to keep track of who knows what. Let's make a list.
People who know Gabriel Agreste was Monarch: Marinette, Nathalie, Tomoe, Lila
People who know Marinette is Ladybug: Alya, Tom and Sabine (new!), Nathalie, Kagami, Félix, possibly Lila (uh-oh)
People who know Adrien is Chat Noir:
(I'm not including Alix, Luka, or the kwamis on here because I don't expect any of that to be relevant. Alix and Luka aren't present, and the kwamis typically don't talk to anyone other than each other and their holders.)
The question of whether Lila knows Marinette's identity is obviously a huge one for this episode. My guess is that she does—the extremely elaborate plan she staged in Secret Protocol didn't seem to be about trying to get Marinette to reveal her secret, but rather about trying to get the Miraculous. And if she hadn't been certain, then she would have almost certainly have taken the opportunity to leave bugs around Marinette's house, like the camera trap she'd set up in Vampigami. So I think it's a safe assumption that Marinette's identity is compromised.
When Monarch found out the identities of Scarabella and Kitty Noire, that was pretty much an immediate victory. Chrysalis has fewer advantages than Monarch, though, because rather than having fifteen Miraculouses and going up against two heroes, she has one Miraculous and she's going up against sixteen heroes. So she has to be much more careful with what she does with this information.
An advantage Chrysalis does have, however, is the knowledge that Ladybug has been lying to everyone about Monarch. If she's outnumbered sixteen to one, then she needs to sow distrust and get the heroes to turn on each other—and she has the perfect ammunition for that. After all, who better than Lila to know how ruinous it is to have your web of lies come crashing down around you? Ladybug relies on Rena Rouge as a confidant and on Chat Noir as a partner, and losing both of them would effectively amount to losing everything.
Let's talk about Chat Noir for a moment. Adrien has been great at keeping his secret identity—he hasn't told a single person, unlike Ladybug who at this point has told several people and has had several more find out. And the flipside of that is that he doesn't have anyone he can confide in, which has definitely been isolating, as we saw in season 4. So one thing that could seriously drive him apart from Ladybug is if he finds out that she's allowed herself to break the rule that he's been following. I don't think it'd be as big of a deal as the revelation that she's been lying about his dad's death, but it could still absolutely be a factor.
Another liability for Marinette once her lie is revealed is Nathalie, who we can recall that up to this point has been probably the primary reason Marinette's been maintaining the lie in the first place. Even if it's not directly spelled out, Nathalie is basically blackmailing Marinette—if she reveals that Gabriel was Monarch, then Nathalie can retaliate by revealing her secret identity. I don't know if this is going to come into play, especially if Ladybug doesn't willingly confess what she's been lying about, but it's still something to consider.
I don't have as many coherent theories this time as I had about previous season finales—but I think the most likely outcome is that Ladybug's lies are exposed and she loses her closest allies, leading to a season 7 where she's effectively on her own.
