My guess is some how Elena is either compelled (by Rebecca) or cursed and forgets the love she has for Damon and Stephen. My other guess is the vampire hunter that figures out the tattoo and gets to "Silas" and the cure turns into Silas, meaning Jeremy turns evil. That way Damon, Stephen and Elena lose everyone they love. Damon may become human, but if you look at some of the pictures they released for upcoming shows, shows him with a vampire bite on his neck, so maybe he turns back.
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