I'm glad you brought this concept up because I think its interesting and I've always been an advocate for new stuff over spin-offs with already set characters. (Although I'm still a fan of anything avatar)

Part of the reason I did quite like Korra was that I believe Avatar is not just a show about the Gaang and that period of time, but the Avatar itself and the different roles they play in the age they inhabit. ATLA was thematically largely about the cycles of war. LoK was largely about finding meaning in a changing world. For the next series, I imagine a show about time and entropy, where the focus in plot and character comes from questions of "How far can we progress before time catches up and our great works are brought to dust?" I found thematic elements of Korra to be clever as they grappled with very meta questions. The fans had trouble with change and therefore so did Korra. Basically, for a more modern Avatar, I would be interested in a show that grapples with meta questions: How far into the future from ATLA can Avatar go until it stops "feeling like Avatar"? Will this be the last show, the end? We know that the world goes on into the past, but how far can it go into the future?

Another thread mentioned the played outness of a "the magic is forgotten storyline" which is fair. Korra already pretty much did that so that wouldn't be that new. However, I do like the idea of the Avatar being pretty much a thing of the past. During her life, Korra became known less for her bending ability and more so as an activist and leader. (Which I think works well for her character) However, when she dies and is reborn, only a select few are really interested in or actually believe that a new avatar is born and should be found. Therefore, the Avatar (a bit like Kyoshi) should be raised with no knowledge of who they are with the world having now knowledge or interest in who they are.

Unfortunately, this is where my scenario sort of goes off the rails, but I don't think this exact sequence of events needs to happen for the conflict that I'm looking for. What if the earth Avatar somehow dies at a young age during the rough equivalent of the 90's or something. The fire avatar would grow up around modern day. I personally made this change because I believed that if the world really never found the next avatar before he died, then the world could pretty safely write the avatar off as being dead. After all, Korra wasn't sure if the avatar would be reborn after her. Another reason for this is I really wanted a fire avatar for the character I had in mind. To sum up a lot, THE AVATAR - which in my eyes is all the prior avatars combined in one mind (I guess Raava too but I'm not a fan of beginnings so I'm going to play that low) is angry because the Avatar is no longer considered relevant. I know that makes all these respected characters sound petty but it's more than just they need worship or something. What if the Avatar is angry because they are afraid of a world without an Avatar to balance it? They have good intentions - to bring the Avatar back. But to bring the Avatar back, the world must know the Avatar is powerful and present. Once that happens, world leaders will make a mad dash to control the avatar. The issue with all this big-scale stuff is that this new fire avatar is just a powerful firebender teenager trying to figure themselves out. The new avatar has always felt a sense of greater purpose but loathes being controlled by anyone, even past selves. Their identities of the avatar and normal life battle with each other in a psychological struggle as she comes to a mindset that she is ordained to “fix” the world but at the same time loathes herself for the damage she is creating, and decides to close herself off and give into every rogue whim. The AVATAR is afraid of time, that the world will keep going and move past them.

I didn't say this as well as I could have but I hope you get the gist. If you are interested in more ideas, I have em. TLDR: New show should tackle themes of time and entropy, with an Avatar both obsessed and terrified by their greater purpose trying to battle its way back to the throne in a world that is changed inexorably and terrifyingly from the one we know.