The thing is when expanding a city is nto to expand too fast.
Look at the RCI Demand, and maybe add a couple of streets with a certain, or a few zones(R or C or I). Then as youexpand the residential slowly, you will then need to put in new schools, medical centres, police, fire etc.

With power, I usually just start with 1 coal power plant, and as the city grows add wind powered ones randomly throughout the city, connected with power lines.

Same with water supply, I just slowly bring the pips out as the city expands, adding random pumps throughout the city.

I love cities with a main land and a island, because I make it, the island, its own little isolated place. I will make residential, commercial, school, medical centre, police station, fire station. Therefore the traffic level on the bridge connecting the island is very low.

With cities with harbours, you need to have a ferry, preferably car ferry, on both sides of the harbour, This is If its a large harbour.
Just say there is the main downtown area on one side, and residential on other side, the people will have to drive all the way around the harbour to get to work, which increases commute time.

Leave space! Don't just expand building everywhere. Leave room for highways, avenues, railways lines, airports, and other things your will need as your city grows.

I have so much more to say, but I really feel like going an playing this game now!