They can easily expand the water area to the right and add islands and other landmasses. Second they can add boats and raiders and enemies who are like apocalyptic pirates.
Driveable boats should be very easy to implement as well. Boats could be upgraded with armor plating, weapons, and customized. They could have a crew and be customizable and built on (use the settlement system to implement this in some way).
Would anyone be interested in this? If not maybe I will attempt to make a dlc.