As I understand it, we are pretty limited as to what we can do with modding.
Create new industries
Create new vehicles
Modify vehicle parameters/costs
This is a good start, but we need access to the game internals to do anything more sophisticated. If the LUA interface was extended and documented, then the community could create a lot more interesting content. For example, the community could create an automatic vehicle replacement widget that everybody on the forums is crying about. Or animated block signals. Or how cities demand goods. Animation of industries. Maybe even the gameplay dynamics could be altered to suit individual demands. The sky would be the limit and this would take the game to the next level. I would be one of the first to contribute to the community. Frankly, creating vehicle packs doesn’t add that much to the game.