I think that Katherine is just the next logical, if there ever was a logical death, in the series. She got Jeremy killed for a cure that she is not even in possession of anymore. Klaus refused to give her the freedom and Silas ended up with the cure anyway. I don't think it's out of the question to say that Katherine is likely to die, but I don't know that Elena could do it on her own. I believe there is no way, not in a million years, that Bonnie would make Katherine truly immortal. She is going to renege on her end of the bargain. That could play into it, should Katherine actually die.