[@AllHollowsEve], so while the possibility of a Dark Avatar is is open, I think that'd be further into the future than what we can plan, and that the Avatar would be most likely influenced by dual spirits. For Rosai's "Vaatu" state (name's easier than "avatar after korra"), imagine Aang's uncontrolled Avatar State in the early episodes, while the "Raava" state would be more like the fully-realised Avatar state in LoK. 

Equivalent power, but one has massive control over one element and very little restraint, and the other would be all four elements in perfect control. Neither would override the Avatar's base personality, and they may even be able to choose which state to go into. There's also that whole aspect of the eyes glowing red (Vaatu) and white (Raava) respectively. 

Our theoretical sandbending avatar would be powerless if caught in the middle of the ocean in Vaatu mode, as there's no, or very little, earth to bend, even with the extreme power. It's basically a trade off. Power for control.

Of course, I could theorise all day. Until we start plotting, none of this could come up. What do people actually want to play?
- New Team Avatar?
- White Lotus Guardians?
- Ordinary Joes awaiting the Avatar?