Okay so everyone is trying to figure out how elena leaves the vampire diaries. This is my theory. Bear with me lol I might be totally wrong.
We all know Damon would rather die then lose Elena, so I feel like when he is given the cure he is afraid human Elena will run back to Stefan and so Damon will hide the cure and swear Bonnie to silence.
However when Stefan turns his emotions back on he runs to Elena telling her Damon has the cure and that Elena needs to stand up to him and decide for herself whether this is the life she wants.
Stefan never forgave Caroline for the evil she caused, and Elena is questioning things After learning about Alaric and Jo's wedding and baby, she begins to imagine what her life might have been had she not turned.
Elena will confront Damon and eventually win out and be given the cure, in turn she will take it and become human.
Damon will want to know if she still loves him and Stefan will tell her hopes now she comes back to him, but before Elena gets the opportunity to make a decision tragedy strikes.
Either Caroline will attack her to get back at Stefan, Klaus will return for doppelganger blood to grow his army, or Damon will accidentally kill her trying to turn her again, this will leave Elena comatose and in need of a body transplant, before she dies.
Bonnie and Damon will team up to find the traveler spell and send Elena into another body, neither caring about the consequences but desperate to save Elena.
The season will end of the cliffhanger of Elena searching Bonnie searching for the spell and a proper body.
In season 7, either Elena will be played by another actress or kept in a coma until Nina Dobrev decides to return, unlikely, so probably a new actress.