I'd like to see more functionality added to existing ships, now engine rooms are appearing, add controls & guages there give us a reason to go there. Add controls to winches so that we can tighten & slacken lines and improve radar & VHF equipment, make it era specific. Add fish finders and fish for fishing vessels. Make sure every vessel has a job to do.
Many want more ships eg Olympic, Britannic etc. I think it would be better to add more types of ship rather than add more vessels to existing types. Add a fire boat that can extinguish fires, add a dredger than can open channels etc.
Also the night time lighting is dire, visibility of lights at night is poor and buoyage is not very realistic. I think these are the key areas that will make important functional differences in how the game is played.