I think he should be an emo b***h.
If they do... yah know, make more series after Korra, they should do like, 1-2 episodes of different Avatars, and just like have a 20 episode season of about 10-12 Avatars (not really one after the other, but over thousands of years) after Korra until they get to a futuristic world and it's the next Harmonic Convergence. And it's a FIRE Avatar, (like, some weird mirror image of Wan) and like, he is very odd, and believes that he should be the embodiment of both good and evil, and willingly absorbs Vaatu as well, while Raava is still inside, and somehow keeps them balanced in the Avatar State, and then touches the portal to cause them to be fused together foreverz. Then from that point on, the Avatar could be reborn as either good, or evil. It would be random, it would be like good, good, good, evil, good, evil evil good, evil evil evil evil, good good, evil...etc.
Would be nuts if that was possible.