As @TheWatcher proposed in another thread, maybe Lily could become the next dark one instead of Emma. Maybe, just as Emma is about to stab a dagger into Rumple’s heart, she has second thoughts because of how it would impact Henry, so she hesitates. Then, in swoops Lily to beat Emma to the punch, plunging the dagger into Rumple’s heart, thereby making Lily the next dark one, and therefore the Big Bad of S5 (assuming there is another season).
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy