For about half the season now I’ve just had a very strong feeling about Regina dying. I don’t know why. And I love Regina! I think her death would have a huge impact on the curse and invite another villain to take her place.
If not Regina, my guess would be David or Mary-Margaret. Those would be the biggest shocks. Still not sure about Belle. Her death would provide Rumplestiltskin with even more motive if she were to die. Though it would suck for them to finally reunite and then, wham, she’s dead.