I only ever watched a few episodes of Avatar, since I didn't have cable. My friends would show it to me occassionally when i'd visit their houses. I liked the show, but it never clicked to me strongly enough to watch the rest of it when I had the ability. I feel I really should. I did dislike Korra though. My friends invited me to their watch parties as it came out, and I watched their excitement turn to frustration as it went on. I think its like you said: Avatar was a simple premise done as well as it could be. Korra just... tried to be more. Deeper relarionship drama. Heavier subjects. More threatening villains. But to me, none of these aspects were enjoyable to watch. I enjoy my dark shows, don't get me wrong, but to me it just didn't land or belong in the Avatar universe. It almost felt spiteful of the source material at times. (I don't remember this being a spoiler as much as it was just a plot point along the way, but just in case: Spoiler Like when they find the corpse of the guy who flew off in the traveling library of the original. )

I would implore you to watch Legend of Korra. It is a great show that tackles more issues than ATLA does.

I really love the legend of korra almost if not just as much as the original series but its incredibly messy, especially the second season to the point i almost dropped the show while watching it and the ending to the first season is a huge deus ex machina, a lot of the issues of lok aren't entirely the fault of the creators nickelodeon treated the shows and its team pretty bad during production also the stuff you were wanting in a s4 was addressed in the comics which are definitely worth a read imo