In regards to a possible "Dark Avatar" not having all 4 bending powers, It may be possible with Vaatu's power to steal the bending powers from others. I mean it's pure speculation, but if you think about it, Vaatu IS basically darkness/disorder/chaos itself.
If the writers DO go in that direction (which I think they will, the Light vs Dark theme is just too obvious, and the villains Korra is facing are not particularly THAT threatening) then they could easily make the idea work, and as Polantaris said, if they wanted to end the battle with the Dark Avatar after a few reincarnations they could easily do that.