It would be nice to be have more granular permission system. I.e. split the permissions by waytype - allow other players to use only roads, and harbors but not railway or airports... Or even set the permissions similar to overtaking options.
I can see that this would be a good thing to have. It is not currently a high priority for my time in light of the long list of other items, nor is it likely to be for some time, but if anybody would like to code it, I should happily incorporate it.