The only reason why I would like to see more modding, is that currently there aren't many interesting cars in LFS (my opinion, obviously if you disagree, then that'll be your opinion...) and so I use Mekanik and LFS Tweak to help model other cars that I would like to see in the game.
I would really like to see an upto date version of tweak that had all of the old S1 abilities, such as changing wheel locations and adding downforce, but only for the single player game. Coupled to this it would be cool to see a few more car models, such as a mid-engined car similar to an NSX or Esprit and a saloon or two.
The one thing I do like about rFactor is the wide range of different cars and tracks that you can drive, which will no doubt help to continually revitalise the gaming experience.
cheers,
Rob