Night racing isn't just about darkness. Darkness is just the most visual aspect of it. Change of moisture levels and temperature drop will have an effect on the tires and the track surface which is already something that lfs doesn't do.
Rain on the other hand is something a lot harder, especially if done well. Puddles, water streams going over the track and different hardness levels of rain just to mention few of the very tricky aspects of it. Just to think about how many years it might take from eric to create some "wet" textures makes me cry. I don't see it beign worth the effort, at least if eric is involved any way. But that's just my personal opinion
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A simple step to begin with would be to have variable track temperatures within sensible limits so the track isn't the same every time. This is still something none of the other sims do and would greatly add a small weather element into racing and lfs. Naturally the tire heating issues need to be solved to take full adventage of it but still it's another quite small step to take with quite big realism/immersion adventages. Later on adding moisture levels and then fog and stuff like that, getting slowly towards the rain conditions would work much better. Even if the actual rain conditions are never seen.
In the end though, night racing is just a novelty and rain is just probably something 95% would not want to do after few laps. More important stuff is needed first and all that blablabla