I added custom trees around the cornucopia and took out all of the other trees. I think this will not only make my map look better, but also make for better pvp in the more open area. I kept trees towards the glass dome and next to the buildings. Reason being I think it helps add to the overgrown theme of the buildings....
....and as for towards the glass dome I added an older radio station and an older home. They both fit nicely, but I think the older home needs some explaining. I decided that it would be logical for there to be a house that was built before the city. It is fairly isolated with a small cobble path leading to it.
As for the sewers, I was not exactly sure how to change them. The only way I could think of altering the pipes would have been to make them green (Like TSG3) or to keep them the same like (SG4&SG2). So I decided to keep them the same, but to just add more "pizzaz" to them. So I made a central area where all of the pipes meet up. This area is also viewable from a giant hole in the streets.
As for the train: I made the color scheme match like suggested, and I also made the shape make more sense. (This is hard to explain, but I am pretty sure you will see what I mean.)
Also in one of the pictures you can see that I made the terraforming on the bottom of the lake smoother and less rough and ugly. I think it looks much more realistic and prettier.
Lastly, I went around to all the interiors and add touch ups. Basically, I didn't add much to them, I just made them make more sense. I also took out all of the random signs.
The last thing you mentioned was the theme of the buildings between the bricks and the stone bricks. I thought it over and decided to keep it the same. I agreed with your point, but in an actual city, there are buildings of all sorts of colors and sizes. So I decided to keep them the same because I thought the variation looked nice.
Thanks for your critique once again, and I hope after this that your rating for my map can go up.