I think it is a good idea to let the game look first in (ex: my documents/Transport Empire/Plugins) and get all the content that was dumped there.
Its like Simcity 4 does. Just one file, and a simple plugin program to activate / deactivate any plugin that is in the folder. I think that the coders who have not the skill to code the game by themselves, they can code such a plugin program. It reads the plugin, and switch it on or off.
A plugin datafile needs:
- Configurations (like on / off)
- Screenshot (if it is graphical)
- Dependencies (packs)
After checking all the dependencies (if it is needed) it will run the game. If there is an error or missing file / dependency pack or whatever, the game should return an error message with the name of the missing dependency (pack).
With this all, the content for this game will be made by a huge comunity. For example, if you like to use USA road textures, you can enable / disable that in that plugin program.