Been rewatching the first season on BluRay and while I know the original premise of the show was one-and-done after the disaster of the motion picture (which had NO business ever being made) because there was little confidence in the brand... but there are clearly some big hooks placed in the narrative.
They could have been conservative and made a series about a new female avatar slowly learning the other 3 elements and then facing some global threat, but they changed it up by making her nearly done with training and the whole Roaring 20s / steampunk nature of Republic City. Sure the plot with Amon was to show a big bad, but it really dredges up the whole notion of discrimination in asking what the nonbending populace thinks about benders.
Then you have the central question of the origins of bending and the whole spiritual world which we touch on some in the final season of the original series and which seems to be the entire focus of this upcoming season. In many ways they are picking up elements of the original story and running with it rather than doing the safe and easy thing.
Finally there is something up with Mako and Bolin. Brothers who have two different bending alignments and sons of parents who left them when they were very young... now we have this whole prequel series about the two of them as youngsters. Something aint quite right with those two.