We've never actually seen much of a mature Avatar. Aang only mastered the Avatar state at the end of the series, and Korra hasn't even bothered to perfect the four elements yet. It's possible that they learn the secondary elements later in their lives. But we do know that Aang could redirect lightningh and bloodbend.

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.)

He never mastered the element of surprise.

You can master deeeeeez nuuutz

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.

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.

Well, to be fair, metalbending was unheard of until Toph invented it. And didn't he use ice when he froze himself in the ocean?

Jack of all traits, master of none.