Do you find cameos from the Gaang/Gaang descendants to be too fanservicey sometimes? I don't have a problem with Tenzin & Lin since they were always an established part of the story, but I find Zuko to be unnecessary. I might be in the minority here bc I know the fandom loves Zuko, but Tonraq, Eska, & Desna could have handled the breakout themselves with little change to the story itself imo. I'm not super angry abt this or anything, but I think ATLA cameos can distract from the new characters.
I’m agnostic on the Zuko question right now. I shall wait until the end of the season to pass judgment on whether or not his inclusion was essential to the plot. Right now, I think you are correct that we don’t “need” him. There are plenty of characters who are capable of explaining the Red Lotus threat to our central cast. Perhaps what having him there does is accomplish is to demonstrate that containing those criminals was a concerted international effort.
There have certainly been appearances by ATLA characters or their offspring that felt fanservicey. Cabbage Guy is up there, as is, quite frankly, Iroh II. As much as I love Dante Basco, he isn’t as versatile VA-wise as someone like Grey DeLisle, so it always just sounded like Zuko’s voice coming out of another character’s mouth. Even though a lot of people liked the character, I think they were right to keep him on the sidelines after Book 1, since the voice issue always threatened to pull focus.
But elsewhere, I think the old characters have been used extremely well. Katara was a beautiful, poignant bridge between the first series and the sequel in Book 1. And her role as Korra’s waterbending instructor makes perfect sense. It also made sense to explain the roles that each of the Gaang members played in the building of the United Republic.
And while a couple of people have described Old Iroh’s return to me as “manipulative,” I thought it was really beautiful and, again, a perfect use of his character. Iroh was the one character who most perfectly embodied the idea of balance, so to have him arrive on the scene to instruct Korra felt honest and necessary. There really was no one else (including Tenzin, who was wildly out of balance himself last season) who could have told her what she needed to hear in that moment. The explanation for how he came to be there, once again, felt believable given what we know about him.
To sum up, I think there are a few questions you can ask to help you figure out whether it’s “fanservicey” or not:
Does the character do something that is integral to the plot or to just to do awesome shit that doesn’t have any real perceivable consequences?
Is it necessary to have that specific character do it? Or is there someone else established in canon who could have served the same purpose?
Are the circumstances surrounding the character’s presence believable given what we already know about the character?
Does the character pull focus from other more essential plot or thematic concerns? Is the story serving him/her rather than the other way around.
These aren’t infallible by any stretch. The answer to 1-3 for Zuko right now is “kind of?” But thus far his scenes have been short and limited to a very specific sub-plot, so it doesn’t pull focus. We’ll see how it all shakes out in the second half of the season.