Marinette's Dream: a Look into Her Subsconcious
Marinette's dream is a lot deeper than just fearing her identity will be exposed because she made the wrong choice with Alya. But that's as good a starting point as any.
Alya
Ever since she became Ladybug, Marinette's been painfully aware of the importance of her role and the heavy burden on her shoulders, something that sets her aside from Chat Noir as she herself pointed out in Reflekdoll. That burden has only become heavier since she became the new Guardian.
The effects her new role was having on her were wrecking her social life and her mental health; eventually leading to Luka's akumatisation and their later breakup as well as her almost drifting apart from her friends, who were also akumatised due to her secrets.
But all that stress was resolved once she let Alya in, right?
Wrong.
While it is undeniable that Alya has been a huge help ever since she learned the truth and Marinette's been able to rely on her, that doesn't change the fact that Marinette has developed trust issues during her time as Ladybug. Even before becoming the Guardian, she still had to make sure the people she chose as temporary wielders were trustworthy and could be entrusted with the mission of returning a miraculous; a mission Alya herself almost failed to accomplish.
But now Alya's no longer Rena Rouge only in extreme circumstances, now both Trixx and the Fox Miraculous are always under her care.
And, even if Alya's proven herself to be a loyal friend over and over, that is still a terrifying prospect: Because Alya's own characterisation gives plenty of opportunities for her to put certain priorities over others.
Up until Sapotis, she was almost obsessed with finding out the truth behind Ladybug and Chat Noir; her first instinct after transforming herself was to ask some personal questions to Ladybug until she gently told her to reel her energy in.
Alya is also especially interested in finding out information, more so if said information has anything to do with the miraculous. Don't forget Sentibubbler isn't the first time Marinette's feared she made the wrong choice with her; she panicked as well in Feast after Alya dibulged an awful lot of information regarding past miraculous holders in her blog.
And now that Hawk Moth not only has upgraded to Shadow Moth, but he also knows the holders' identities, a fact he already used against them in Optygami, Marinette's more on edge than ever.
Nino
Believe it or not, Marinette's fear that Alya might've told Nino is perfectly justified.
Alya and Nino are a strong couple who share many things: interests, they're the first ones to know each other's secret identities, they even got akumatised together... They're very in sync, which is something Hawk Moth knows and has used to his advantage.
In Heroes' Day, Rena's akumatisation into Rena Rage then led to Carapace becoming vulnerable enough to be akumatised himself. And in Optygami, SentiNino became the perfect mole because Alya would have never suspected of him.
If that weren't enough, Nino is the one person Alya has ever "betrayed" Marinette for. Despite being sworn to secrecy, that didn't stop her from telling him that Marinette is in love with Adrien back in Animan. Sure, it might've been to get him to give up on her, but that doesn't change the fact that she betrayed Marinette's trust to some degree.
Chloé
Now this is the most interesting part of the dream.
Of every temporary holder Marinette could've dreamt about (say, Kagami, who could've accused her of sabotaging her relationship with Adrien in Heart Hunter), she subconsciously chose Chloé.
And you might be wondering; why?
After all, even if Marinette could've done things differently, Chloé was still mostly in it ("it" being the superhero business) to stand out, be popular, and ultimately betrayed Ladybug either way solely because she couldn't compromise and just had to get her way with things.
The answer is simple; because Marinette feels guilty over Chloé's situation.
Even if Chloé turned out to be more trouble than she was worth, Marinette still cares about her to some degree; she feels responsible for her. Being Ladybug and therefore Chloé's idol has allowed her to see her most vulnerable side, to the point she even shows sympathy over a bully who's tormented her and her loved ones in the past.
Deep down, Marinette blames herself for the hole Chloé's dug herself in, because it's a direct result of her secret identity policy. Is she entirely at fault? No, of course not. Chloé dug her own grave. She chose to betray Ladybug and to abuse her position as Queen Bee to be exceptional rather than as a chance for personal growth. But Marinette is still a sympathetic person who hates seeing others suffer.
Remember she got genuinely worried when she saw Lila approach Chloé. Because even if both are bullies, Marinette is perfectly aware that Chloé is the lesser of two evils and that whatever Lila had told her, was meant specifically to hurt her.
Chat Blanc
Even though this is supposed to be the most surprising part of the dream, it's also quite obvious, don't you think?
In the end, Chat Blanc is the ultimate manifestation of Marinette's biggest fears.
Marinette has some clear trauma resulting from her experience at Chat Blanc's timeline. If finding herself in an apocalyptic world weren't bad enough, Chat Noir, her kitty, was both responsible for it and deeply traumatised himself.
And what does she know about the reason behind it? That it was a result of them knowing each other's identities.
That's literally it.
She never learned Chat is Adrien and the two were actually in a loving, stable relationship. She never learned the true factor that ruined everything was actually Gabriel. She never learned Gabriel was really Hawk Moth, or that he was so obsessed with his own goals that he was willing to sacrifice his son's sanity, causing the world to burn up in flames in the process.
She knows nothing of it.
All Marinette knows about that timeline is: She and Chat Noir revealing who they are to each other equals total destruction.
Summing up, Marinette's dream is a perfect and concise summary of her deepest fears. It's a manifestation of numerous traumatic and/or stressful experiences we just had never seen the aftermath of. And I, for one, feel like I have to applaud the crew for that.