I haven't downloaded it but I reckon the following would be a good start.
- Add lighting.
- Better textures.
- Re-import (or redo completely) the Nissan Silvia.
- Better effect for the implosion of the buildings.
- More details like lamp posts, semaphores, road markings, etc.
I am aware that this is the list you wrote yourself. Basically, I don't quite understand why you're asking for input when you already know what needs doing to it. Keep working on it and maybe ask for feedback a little further down the line.
It's a de map, based in a city. While it's not overly original its a working formula so just keep going with it.