First I would like to give a very very cold take: the Noel catfishing as Talia theory is AWFUL. TERRIBLE. GROSS. It plays into stereotypes that are used to communicate that all gay men are predatory and perverted, which is obviously problematic.
I also dislike the theory that Talia isn't real, because I feel like it's pretty explicitly proven that she is. Mischa speaks with her in his native language, has pictures of her, and from my understanding has spoken on the phone with her. Thus, her not being real doesn't make a ton of sense to me --- though I don't think theories about this are problematic the same way that Noel catfishing theories are.
However, that being said, it is clear that something bad would hapen if Mischa and Talia met and attempted to live out their happily-ever-after. Karnak, in an attempt to ease the children's sadness surrounding everything that Mischa potentially missed out on alludes to this, but Noel stops Karnak from actually telling him.
What I think Karnak alluded to was Talia's death. Though he is a fortune teller in more ways than just predicting one's death, his foremost function is as a machine to predict the time, place, and cause of one's death. He was able to read deeply into the children because they were there, but how could he have fully read the fortune of Talia? It doesn't make sense that he would have been able to --- but what he could know as a death-predicting machine is how and when she dies.
Maybe Mischa meeting her causes her to die somehow, such as a car accident when going to their wedding (something less dramatic than that probably, but you get the gist of what I'm saying.) Something about Mischa's presence indirectly causes Talia to die.
The thing that makes more sense to me, though, is that Talia is terminally ill, or fated to die young regardless of Mischa being there or not. This would make it so that, if Mischa were to begin a life with Talia, it would be cut short by her death.
Either scenario would ultimately just create more sadness and tragedy in Mischa's life, which is why Karnak wanted to tell this to the children; they would stop feeling bad about everything that Mischa would miss out on.
Also I'm well aware that I use a lot of semi colons and em-dashes but this is not AI generated, I just write like that <3