"They're erasing an essential part of Katara's character by having her merge with the Blue Spirit; they're erasing one of her most important connections to the love of her life. THIS IS ALL JUST A ZUTARA FIC, THEY WEREN'T SO FEMINIST AFTER ALL!!"
The second season hasn't even premiered yet, we don't even know if these are the same scenes, we don't even know if she is Katara, or THE SPIRIT HERSELF, because I remind you, The Painted Lady is a real spirit within the Avatar universe.
Even if she is Katara and she joins Zuko, it wouldn't preclude her from appearing in the third season and performing the same act as in the series. However, I don't see the Blue Spirit joining her as a possible scenario. For starters, Katara learned about this spirit in the village she protected. It would be very strange if she suddenly decided to wear THAT outfit instead of a cape or the uniform she wore in TSR to go on a mission. That outfit is anything but sporty, and she doesn't know anything about the Painted Lady yet.
UNLESS she first does what she did in the original series, and then, having her alter ego, she decides to use it for the supposed mission she will carry out. This proves they won't delete the original scene, it will only happen sooner than expected so she has a heroine disguise.
Furthermore, this blessed mission that everyone is clamoring for is that she's on her way to rescue Aang in Ba Sing Se. YOUR DYNAMIC IS RIGHT THERE, DUMBASS. The roles are reversed, the girl saves the hero. If they were eliminating their blessed, all-important bond, we wouldn't see Katara so determined to infiltrate Ba Sing Se and risk her life to RESCUE AANG. She didn't do that in the original script because she was sick, she is now determined to save "the love of her life".
The fact that she's joining the Blue Spirit to save Aang only confirms what you've all been saying this whole time, right? "Katara and Zuko would choose Aang over each other"? Wasn't that what you kept repeating to discredit Zutara? It's coming true now, so what's the problem?
I think you guys are the ones creating the Zutara fanfic; even we haven't come up with such ideas. Kataangers exalted to confirm that Zutara will be canon in NATLA, and Zutarians furiously trying to assure why it won't be—it's a scenario of pure fiction, something no one ever imagined.
My theory: Fan service, pure fan service. That scene might not even happen in Book 2, what I have understood is that the actors have also filmed some scenes for Book 3. It could just be a preview of Book 3, a post-credits scene from the season finale. They might not even be the same scenes, but they still put them together to generate what they're so good at: driving fans crazy and attracting viewers with the most famous ship in the fandom. It doesn't matter if the reaction is negative or positive; If there's a reaction, more people will be eager to see it. That's how it works.
They did it in the trailer for Book 3 of the original show, so why not do it again, since it worked so well for them? That strategy worked so well that people now just treat it as a coincidence, they completely washed their hands of it. They knew what they were doing when they cut the scene of Katara pulling away from Aang's kiss to the scene of her hugging Zuko. They knew what that would cause: how excited the fans would be to see their ship canon, their theories confirmed.
And then to mock them to their faces.
Except from what I can see, it'll probably be the other way around. Katara will likely end up alone, without any interaction with either Zuko or Aang (or conversely, Kataang will be canon). And you guys who were on the verge of a heart attack over the ships will just look like idiots for going overboard.
This might not age well; Zutara might become canon. So what? I might not be right, but I'll have my ship. Either way, it'll be a win for me.
The only thing I've managed to confirm with this is that those complaints about "Zutarians don't accept canon, they can't get over it" and "They're annoying for not accepting things as they are after 20 years" would have been a perfect fit for Kataanger fans if Zutara had been canon, because I'm sure that today they would be just as intense as the people they criticize. They would be the ones making "anti-Bryke" posts for putting an indigenous girl with her colonizer, they would be the ones commenting "Kataang should have been endgame" on every Zutara video that popped up on their social media, and they would be the annoying shippers that simply "can't get over it."