...What? No shit... :facepalm:
In the first series in season 1 Aang takes the form of Roku (the Avatar before him) multiple times and he uses firebending, this was before Aang knew firebending himself, but he was in the spirit world and Roku told him that there were Fire Sages waiting for him outside and he could help him fend them off and escape after exiting the temple.
I'm saying it would be badass if Korra did the same but took the form of Aang (the Avatar before her) and used airbending.