Alright, I haven't created a map yet since I'm a fan of group projects and I believe this entire city can be freeformed to some extent; that is, within the rules we decide upon.
What I did make is a basic example of road/lot and how they can be connected.
The system I had in mind works by taking 10x10 as the standard (and strictly speaking, the minimum) size of a 'piece' on the map. The attached image shows this, where one stretch of road is 10x10 (3 sidewalk, 4 road, 3 sidewalk)
It's important to note that the sidewalk is 1 block higher than the street, and that the street and sidewalk are made up out of cobblestone, and the edge of the sidewalk out of 2 stone slabs (as coloured on the map)
All lots (player and public) are at sidewalk level, making the roads the only -1 height areas in the city.
On the top left side of the picture you see a standard-size lot, with 2-wide grass around the house. I suppose you can plant flowers there - I would keep trees in a special park-style lot which is basically 10x10 green.
As you might notice, this lot can be connected to any stretch of 10x10 road, which makes the whole ruleset simple to maintain: if you end up with a house that's longer than the stretch of road next to it, you made a boo-boo
The bottom of the picture has an example of how to use the pieces together. The road has a turn in it, the red lines mark the turn section as a single 10x10 piece, and using your imagination you could form an entire network of roads using 10x10 pieces in this manner. The example also shows a 20x10 player lot, with two entrances. I would suggest to make the maximum player lot 20x20, which ought to be plenty of room for building a reasonable house. Maximum height could be maybe as low as 10, in order to ensure you're not constantly faced with a wall of stone all around. I guess exceptions can be made for special houses or towers. Being an ever-collaborative Dutchman I like the idea of informing your (future) neighbours of build projects and whether or not they object to certain aspects of it.
I would further suggest that every player contributing to the city is responsible for connecting their own lot to the road network without breaking the 'rules'. As a bonus for building roads I thought it would be a neat idea to let the builders name the street they made by placing a sign on the corner of the street at 3 blocks height. In my mind we'd have an amazing city where people have adresses consisting of street names and numbers, but I could be overthinking the idea
With the explanation out of the way I suppose you all understand that a map is not really required. I'd personally like to see what we end up with if we start freeforming whilst maintaining the 10x10 rule. If anyone has a good, reasonably flat piece of land in mind we could start working at any time. All we need is a couple of perfectly sized roads and connecting lots (pieces not being 10x10 could ruin the consistency of the city in the long run) and the project should take shape nicely.
Any suggestions are welcome of course, it's not my project after all - it belongs to the group of players active on the server
Also, MS Paint yo!