My suggestions:
Mods should be moderated to a certain extent, reviewed by the devs themselves or anyone assigned the task, and they should pass the following:
- content of mod
- licensing issues (If it needs a license, fail)
- working properly (No bugs visible)
- For cars, realism (Is it a 2000 HP, hover car? If it is then fail)
- File size (Nothing too big)
- Does not alter any of the current content
That's what I can think of for now.