Since there are so many spoilers floating around about a major death coming up everyone is thinking that most probably it is going to be Damon or Elena who would either die or become human again to break the sire bond.
But I have another theory.
Tyler (and other hybrids) bonded to Klaus because they were grateful to him for releasing them from the pain of turning every full moon. So, the way to break that sire bond was to turn until they don't feel the pain anymore.
I think Elena bonded to Damon not only because she was in love with him before she turned but because she was hiding from those feelings and denying herself that love that her and Damon have for each other. So, the way to break this bond is not just by one of them dying or turning human but if by accepting those feelings. And not just by Elena but also by Damon.
In 4x10 Elena realized and accepted that love but it's pretty obvious that Damon doesn't believe her. I think the day he accepts it as reality, the sire bond will be broken.
So, that's my theory…..I came up with it mostly because I want the writers to surprise us for once and not go the predictable route.
