Shaheen is most likely going win again, even if Trump flipped New Hampshire she would still win. Shaheen is a more established Senator than Hassan, and would beat Kelly Ayotte, Frank Guinta, or any Republican thrown at her, by at least 4-5%.
Governor Sununu could beat Shaheen, but as he has already declined to run, I don't see the point to looking at polls pertaining to a potential senate campaign from him next year.
Many times, politicians decline and look back at the race later, for example, early on, Joe Biden repeatedly declined that he had interest in the race.