Honestly, I don't know why this is not a thing already...
Allow the players to choose which mods they want enabled or not (but limit them to vehicle and skin mods, to avoid objects being weirdly placed). Could probably be a menu that displays the server mods the player is allowed to toggle between on and off.
It's very frustrating that if I wanted to use a police vehicle mod, I'd have to disable all server mods, meaning a lot of skins would look stupid. I wouldn't see any other vehicle mods, and, as you can see in the screenshot below, some objects are changed server-wise too.
Server mods on (Link)
Server mods off (Link)
I don't know if this has been suggested before, but I couldn't find anything related to it.