After reading about it on my Book Four rumors thread, I found out that Kuvira has some scenes reportedly from Book Four depicting her as sinister.
While, I personally believe those images might have just been deleted scenes from the season, I will entertain the thought of Kuvira being a big bad in the next and finale season of Korra.
First, some backstory to this rumor:
Kuvira was seen in some promotional material for Book 3, depicting her with a sinster grin and look, many theorized that she'd be one of the villains of Book 3, something that obviously didn't happen.
Kuvira is played by Zelda Williams, daughter of the late great Robin Williams, and has had some signficant scenes in the series already, first introduced from a metalbending routene and then helping save Korra from the Red Lotus, her biggest moment, however, is being the one to save Tonraq from falling to his death and patching him up. But something odd happens while she's patching up his wounds.
She tells Tonraq her name and then, some really sinster music begins to play after she states it.
It's kind of odd how she would introduce herself in such a way at the very end of the season and then have such a unusal track to go with it. It's almost as though Bryke was.... hinting at something.
Something is obviously up with Kuvira, and I think it's possible to say this:
She might be the villain of Book Four.
Along with the supposed Book Four clips from the teaser and her introduction, we can see somethings present in the clips that might hint at her postion in season four:
For one, she's simling sinisterly with a group of people sitting at a table (Most likely Zaofu Metalbenders), it looks as though she might be planning something or discussing something, it's possible that the villains might very well be the Metalclan's guards with Kuvira leading the way for whatever reason.
We can also theorize that she is apart of the Red Lotus and so are the guys behind her.
And secondly, we have another clip from the teaser of Kuvira seemlying attack someone, we've seen her fight against the Red Lotus, but the background of her fight scene is all wrong, it looks as though she's fighting in a city.
I think it's safe to say, rather or not she'll be the villain or one of the villains, that Kuvira will have a large, and possibly deadly, part in Book Four.