So, I just got finished reading The Promise part 1 and 2, and I have to say I am thoroughly impressed with it. I thought it was great, the story, the art, and really captured the essence that is Avatar: The Last Airbender. It is also a tie-in to A:TLA and TLoK, and so will the upcoming The Search trilogy. I think there's also an opporunity to focus on TLoK mainly, in a graphic novel.
There's the first episode of Korra. It skips from her being, say, 3-5 years old to then 17. There's a lot of opportunity there, (yes, opportunity again), to create a graphic novel, much like The Search, where it has a story of her training of mastering Water, Earth, and Fire. I think this could be a great part of the lore, especially if you add in these characters left out, her parents Tonraq and Senna, as well as Katara, and numerous other new characters. I know that Tonraq and Senna will be very much included into season 2, but I believe that a graphic novel on this would be a great way to fill in the gap as well as further develop some characters.