Think about this for a minute.
Water is used by the Avatar but Ice is never used
Earth is used but metal bending isn't
Fire is used but lightning is never used but can be channeled.
Air as far as I know has no 2nd ability.
To master these elements surely the Avatar must master the 2nd abilities of the elements after all all the "Masters" of each Element have mastered the secondary ability.
Should we see the avatar learning to bend Ice, Metal and lightning or is that asking to much?
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What? Ice was used consistently throughout the series. Lightning was created out of thin air by Azula, Iroh, and Ozai. Metal Bending was used consistently after Toph discovered it. Lava was used by Sozin and Roku. There were Sand Benders, Mud Benders, and Slurry Benders, so it only makes sense the Avatar could use those as well.