If you’d like to make your own Sim City mods, here are some tools to get you started modding Sim City!
S3pe is a Sim City Package editor. Sim City uses the same sort of package files as are found in The Sims series. They are therefore highly moddable, even if EA has changed their minds about how much they want Sim City to be modded.
This is a program that allows you to view Sim City package files. In case you wanted to know what they looked like, on the inside.