Well, since Korra's fully realized, we'll undoubtedly get to see some very advanced Airbending. What I'd like to see in that arena is her sparring against Tenzin, or going into combat together, just to see the creativity the two of them could employ.
And if they keep making new Avatar Series, they'll certainly have more challenges on their hands. They'd have to keep upping their game to epic proportions. Though, admittedly, it would be quite awesome seeing Korra as some Earthbending Avatar's spirit guide (she definitely has an Earthbender mindset).
Speaking of what's next in line, I wonder what the next book will be? Back to Water? Or maybe continuing Air? What if it's a secondary element?
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)