Avatar could come full circle and do another show about the next Air Nomad Avatar (someone who was different from Aang) and even another one about the next Water Tribe Avatar and so on and so forth and I would not protest. 

you know how doctor who’s been around for 50+ years because the doctor’s regeneration gives the audience a fresh face, personality, and plot to keep going with?
yeah, avatar has that advantage too.

The only minor issue is the series gives the impression that technology is progressing at a pace roughly comparable to our own. Given that Avatars live extended lives (Aang canonically spent most of his life-extending abilities keeping alive in that ice block so he “only” lived to be like 100 or so allowing Katara to outlive him), and given that in Korra’s era it’s roughly (emphasis on roughly) the early 20th century, the next Avatar could very easily be living in the equivalent of our future.

So… near-future Earthbender, or maybe cyberpunk Earthbender? Possibly followed by post-apocalyptic Firebender and then post-post-apocalyptic (yes, that’s a thing) Airbender? Alternately, Firebender in space and Airbender in Space Opera.

Personally I’m not 100% sure that’s what I’d want since none of that feels very Avatar to me, but I’m sure a ton of people would love that. Also, they could go back on their implied technological advances if they wanted to, or go the High Fantasy route and slow down technological progress and blame it on magic (aka bending).