I think Aang should be more than just history in Korra. Korra should, at one point or another, speak to Aang spiritually the same way Aang did to the Avatars before him. I also think Korra should have another flashback that shows Aang's final moments before passing. It would be a heart wrenching scene, but I am very interested to see what his last words would be. I just hope that they don't pick an unfitting voice for older Aang. I would be crushed if his mature voice was not like him.