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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I'm inclined to think that if an Avatar managed to study and work on their abilities long enough, that they are very well able to use advanced bending techniques, but the only Avatars of whom we as viewers get to see enough of a chunk of their lives to know what they do, are all young, and only just getting into multiple-bending styles amidst eras where they don't exactly have a whole lot of "free time"/ years to learn the hard core stuff. For all we know, Aang and Korra do eventually master metal, ice, lighting, whatever bending after their respective series have ended.