Personally I think the adaptation of ATLA is a really problematic idea with so many issues that come with it. The length of the shows, how to fit in the storylines, how similar/different should the show be etc...Similarly I think adopting the Legend of Korra to a live action would be a disaster because of just how much inconsistancy and how overdone Korra is as a subject for a show. Personally I think going deep into the past for a prequal/independent story would be great. It could be well beyond Yengchen's time. The show would be free of any restraints it might have with storylines and whatnot and they would have another shot at making something that would reach the level that it did in the original show. The repeat we all know is just not going to live up to the show, it's just going to be a letdown. We can only hope that after they are done with the original series (assuming they get renewed for all three books), that they will eventually return to Avatar in the years after/some producer/director who grew up with ATLA will revive the series for the next generation with a new storyline.
A truly original, animated story, sequel or prequel, would be the best for me.
But, this is the same reason Disney is remaking their classic animation as live action films instead of spending that money on new ideas. Cash cow. Live action remake of ATLA is going to be the most profitable for Netflix.