My Theory is Avatar-State Korra will be the new villain. After not-Amon almost ending the Avatar cycle, it has triggered events in the spirit world where the Avatar's era is coming to a close.
So Avatar Korra is trying to fight Normal Korra and if Avatar Korra wins, she gets full control of Normal Korra's body and will unleash an Avatar State of tyranny via might makes right.
Then Asami breaks the power Avatar Korra has over Normal Korra's body by exploiting her body's weakness. Asami grabs Korra around the neck and attempt to break her control by feeling her tonsils with her tongue.
During Korra's bout of physical impairment, it was Asami who was at her side through the whole thing. In her weakness, she tended to her, cleaned her, and caressed every intimate inch of her body.
The euphoric shock briefly loosens the control Avatar Korra has over Normal Korra's body. The two lovers exchange good-byes, Asami knows what to do. Tears in her eyes, she grabs Korra's left breast and begins to pump deadly amounts of electricity through her heart, killing her lover, their secret dying right then and there.
The nations mourn the loss of their avatar, Asami is sent to be executed, as the gallows loom ahead, a smile creeps across her face. She will be with her soon.