The whole night thing makes sense. Not sure fluff-wise, if it matches up, but in the movies the battles of the last alliance were in the dark, maybe because of a dark cloud over mordor, or maybe because it was night, either way, night fighting rules should apply. Same thing for the battle of the hornburg. Battles in a thick forest like fangorn, mirkwood and lorien, i would presume to be dark, as well as ones under the mountains like moria.
Instead of doing an entire expansion, you could condense it down into a scenario. By that i mean, clump rules into a couple more generic rules, and then pick a few that you like. Theres probably 4 main categories:
Day benefits (shooting), day negatives (orcs and such), night benefits (orcs and such), night negatives.
If you wanted to do an expansion, it might be a good idea to write scenarios for battles that are fought in the dark, alot of which arent in the core rulebook (all of the battles in moria specifically).
By extension, you could try to include weather conditions. Rain could be the most common, but other things to think about would be snow (the tops of mountains) and heat (open plains, the deserts of harad, wastelands). Rain would make shooting pretty much useless, since the water would ruin the composite materials that make up bows and the fletchings of arrows. It could potentially disrupt movement too. Heat, would effect things that are more heavily armored more-- armor that gondorians wear would cook them, wheras the haradrim have light armor and head coverings.
Just some thoughts. Good luck with whatever you decide to do. I look forward to seeing the final product.