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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Aang has used ice before. He is able to redirect and manipulate lightning so that counts. Never seen him metalbend but he might have picked up the skill somewhere in his life. The secondary abilities for water is blood bending,which he can resist, and healing( which one needs to be born with the ability.)