I believe it'll be Stefan figures out that Katherine is pretending to be Elena, she could care less about anyone else in Mystic Falls, but she wants to win back Stefan, no way she can hide her true nature from him forever.
After everything we've seen? No way this charade will change Katherine.
I don't believe Katherine will be a better friend to Bonnie and Caroline than Elena.
Since another hiatus is coming after the next episode, I believe we won't see Elena in any form the epiosde after next one, which is after the hiatua.
The writers are apparently adamant in keeping her and most main characters alive, so I have to believe Katherine is going to survive, again somehow.