Modding is a really tough subject. I have all the "Major" sims. GTL, RFactor, GTR2, and LFS.
Of all the sims, LFS is by far my favorite. It just feels better then the rest in my opinion. And not by a small margin.
I really do not think that adding cars will be a big factor to me, but being able to add tracks would be huge.
I love Limrock Park, I have raced there a few times in real life, and would love to race it in LFS. A Fox on Limerock would be great. I am sure there are other tracks that would also add a great deal to LFS.
Adding cars is not as big a deal to me. The reason is that i have found that most sims do not take into account enough paramiters to make it 100%, so in the end, we do not have an exact copy simulated, but an interpratation of the real thing. And to that end, the cars in LFS are more then enough. Like i said, they just feel right.
Perfect example, A few months back, my wife bought me a "Nascar Experience" (For the record, i hate Nascar) What that entailed was a day of training, and 10 hotlaps in a Nextel Cup Car. Very fun!! Well, a few weeks ago, my Team wanted to try a new Nascar Mod on RFactor. It looked great, it sounded great, the tracks are fantastic. the car feels nothing at all like what i drove a few months back. Not even close.
Left a bad taste in my mouth.
But a fox at limerock still sound great!