In a recent interview with Entertainment Weekly, the OUAT executive producers confirmed that Regina's dream sequence in episode 21could is indeed a foreshadowing of Regina's fate. Interesting!
Is Regina’s dream foreshadowing? Or was it just a dream? If she were to have Storybrooke turn on her, is that how it would play out?
KITSIS: I would say that it was definitely foreshadowing in the last episode, and I think that if you watch the finale and you go back, there will be a few things in there that will make people go,“Ahhh.”
http://insidetv.ew.com/2012/05/08/once-upon-a-time-season-finale/
"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