Like many others have pointed out, too much content will spread the community apart too thin. And user made content most likely wont live up to the quality LFS tracks currently do have in terms of smoothness and bumps. (The collision mesh of LFS tracks have alot of detail in the form of bumps)
Not only that, user made content overall will cause inconvenience with multiplayer. Either you would have to search for every mod on every server, an effort I am trying to avoid and I would hate for LFS to take on this horrible method. Or you would have to wait each time you join a server for the game to automatically download a bunch of mods, only to end up with a brand new low quality car which you don't have any setups for or any pratice with.
Like I said in your other thread, LFS is about the driving experience, not the content. While real life tracks are fun to drive solely because you know they exist in real life, it doesn't necessarily change the overall sensation of driving on a fictional track with a similar style in corners and size.
The only thing I wish for with LFS is for a few carefully selected content released with the idea to cover more "roles". For example a thin gravel rally course running trough a forrest with limited grip and alot of blind corners. A mountain course with alot of tight hairpins and drastic elevation changes with steep edges on the side of the road to crash down.
I also agree that nordschleife would be a pretty awesome addition but it would have to be a careful scan to live up to the quality of the other LFS tracks.
But to sum it up, don't open LFS for modding.