Dark Avatar Theory
While I think it would be a fascinating concept to explore a dark avatar, I also don’t want to have it be the series finale. 1. That’s so predictable 2. We’ve already seen the Azula v Zuko sibling rivalry.
Additionally, I think it would be incredibly illogical for Nisha, who is guarded and trained by the White Lotus, to be an antagonist. At their core, the group symbolised unity and peace among masters of all bending types. Unless there was a major corruption — like a Red Lotus member infiltration, I don’t imagine pitting the sisters against each other has any real significance beyond a season.
That said … I’ll admit. The dark avatar would be an interesting plot. However, unless there’s a retcon, we know the Lion Turtles taught Raava the elements, and she fused with Wan because he alone couldn’t hold all the elements. Vaatu did NOT learn the elements. So, while he could imbue incredible raw strength, he cannot bend nor pass down the ability to master the other elements. This isn’t to say Vaatu hasn’t learned new tricks in his 10k years but it multiple all-bending avatars aren’t canonised (yet).
I am curious though, how the show will handle the true avatar reveal. So far, we know Korra is the only avatar to have learnt any bending besides her native water early. Either the White Lotus is only expecting Nisha to train with Earth until she becomes “of age” — whatever that now means considering Roku was the last avatar who had a defined age 16 start — or she can bend more than one element.