Time for my predictions:
Trains: guaranteed.
Police: I hope mountain or desert.
Exploration sub theme: hopefully either desert or cave.
Great vehicles: a new tanker truck, camper van, forklift and delivery truck, a mail van would be nice, and finnaly, either an ice cream van or hot dog van.
New themes: hopefully hospital (with large hospital) a new harbour or mail theme. Farm isn't that likely.
60th town plan anniversary set: a replacement to Lego city's largest set (60097) - a mall. It would have the shops of town plan and would have between 1000 and 3000 pieces.
Those are my thoughts on Lego City in 2018!!! :)