The city mockup looked nice, but I needed infinite random cities. Let's try procedural generation! The mockup was created using actual Pico 8 8x8 sprites, so buildings were already modular. I just needed a few more windows (open and closed), doors and roofs:

I used five colors: wall, dark, light, door (these are the three that would change programatically), glass and glass detail. Using PAL in Pico 8 you can swap colors, so with just those blocks in the image and a few color sets (i.e. {4,9,10,2} <- dark, wall, light, door) I would have buildings with different colors.  Lots of sprite space saved!

In order to have a building we need some numbers:

• width (minimum 3 tiles, maximum 5)
• height (minimum 4, maximum 9 for front buildings; minimum 10 and maximum 12 for back buildings)
• color set (wall, dark, light, door)
• window, door and roof sprite.

So a building is just a bunch of tiles: roof, windows, ground level, plus some details like not adding windows at certain floors from time to time, just to add variance. 

Cities are just a bunch of buildings following these  rules:

• Cities have between 3 and 5 buildings.
• There's a road or a railway. No water, yet, nor bridges.
• There are 2 or 3 buildings on the background, the rest of them on the foreground.
• Buildings on the same plane can't touch.
• Buildings can't have the same color as nearby buildings.
• Buildings on the foreground can't be tangent to buildings in the background (sides can't touch)

It took a while to come up with all those rules, but here's the final result. Tons of cities!