I'm guessing that we haven't seen the last of Hugh Armitage. Demelza could even be thick with child(which I'm guessing she'll get a back alley abortion to cure) by the time they move to London, driving Ross further into the arms of another. Also, Elizabeth will be in London, and she's been getting more and more dark as the show progresses.
I'm guess Ross and Demelza's story lines will separate for this next series. I don't think we'll see much of them together. If we do, it will be either fighting in private or a tense feigning of happiness in public to save face.