I'd like to see detailed areas, ports and harbours with all the vessels local to that area released as a patch/add-on or new release on disk/download that you can purchase, this way as a new area is released the developers will have a steady income to develop the next area and it's associated vessels.
For instance Dover-Calais with all the ferries, tugs and other vessels that use the two ports, you could even include Dunkirk as the main commercial port and have all the navigational hazzards etc for these ports, like the Dover straight with AI ships using it and the ferries trying to cross and the shallow water and sand bars in the area.
I'd like to see tides & currents for each area, try getting a ferry into Dover through the narrow entrance when the fast tide is pushing you sideways as fast as you are making head way.
I'd also like to see more detailed bridge consoles and controls with each ship having the same layout and equipment as the real vessel it has been modelled on, as in other simulators like MFS where each aircraft has very different controls and they are modelled on the real aircraft, the same would be nice here too, so you can't just get in and take it for a spin you need to find out what the controls do and have trials missions that you have to pass before you can do more complex missions to show you can handle the ship in different situations.
As well as some other items mentioned in other posts such as windshield wipers, working radar etc it would be good to have port control/VTS radio that you can either control yourself or is AI or online with other users taking the part of port control, giving you information and instruction on when your vessel can leave/enter port, you have to ask permission to leave your berth, it's almost another sim in itself.