The city should have evidence of growth. Big cities aren't made, they're grown.
What do I mean by this? Well, consider:
When the town was created, the houses were probably built wherever the inhabitants felt like. The streets probably were just originally paths where farm animals were taken to market. There should be a portion of the city that has meandering streets and houses built in a chaotic pattern.
Was the earlier town wealthy? Then it must have had a wall, to protect the town. The area that was protected by the wall would be packed tight, as space would be a premium. Eventually, the town would outgrow the walls, and would sprawl beyond them. The walls would then become obsolete. Wha happened at that point? What happened in Europe's history is that the wall would be torn down for building materials, and the resulting clear space would probably be used as a circular road.
Are there governmental buildings? In our own history, there was an age of enlightened (or at least, organized) government. There'd be an area of big governmental buildings, so that the government would be centralized.
How about some late-term city planning? In our own time, when the Renaissance hit, cities were developed according to plans. Just look at London, after the fire, at Paris, in the time of the Sun King, at Washington D.C., and especially at St. Petersburg, where the city was built from scratch. What do you see? A huge emphasis on geometric shapes. You have plazas, boulevards, streets going out in all directions. You have planned beauty.
So don't just draw your city from scratch. Start somewhere, and decide it is the original town. Take into account the changes to what is already there, and the addition of new stuff. As you go along, decide _why_ things changed. That area that's in the middle of the old town that for some reason now has a huge church? That happened when there was a huge fire, and the church bought the land. The big mostly-circular street? The remains of the first wall. That cluster of large buildings? When a strong leader centralized government bureaucracy.
Just some thoughts.
