Another thing that should be altered is the how the advanced bending disciplines work, because in this quest all those skills are simply a secondary bending skill tree to start leveling up once we unlock them by advancing in the main skill tree, but in the series it is actually the other way around.
To unlock any of those advanced abilities is very difficult, but once they are unlocked things change. All the benders that unlock these advanced abilities are capable of using them with a similar level of skill to their main bending ability in just a few days/weeks, an example of this is Bolin lava bending and Korra metal bending.
I don´t really know how this can be balanced, the best idea I have is to reduce drastically the TP needed to upgrade the advanced bending, in the levels that are below the maximum level in the main bending skill, and also to increase the TP of the advanced bending that is above the main be bending level...