With a mod (Martigens Mutants Mod, I think?) I had these glowing ghouls that only came out at night (Night Ghouls) and they roamed the countryside in ~packs~. First time I saw them, I thought they were fireflies (corpse flies, actually, another modded-in critter, I think).... amusingly, I thought it looked a little like a rave, with reds and greens and blue colors... then I saw them move... and then I saw a mole rat family get decimated and then they turned to see ME... and they run ~wicked~ fast. Scared the beejeezus out of me.
Having "nocturnal" foes would certainly be a great addition --- I would not expect to see regular raiders tromping out in the wilds at night, but monster, sure. And maybe night-vision-equipped mercenary hunters (android-hunters?) could be out at night only. As much as I do enjoy the night, for practical reasons, I'd prefer the actual PITCH black night to be pretty limited (perhaps influenced by the weather --- cloudy night = darker night for longer). With starlight, moonlight, and the odd streetlight / campfire and night-acclimatized eyes, some level of brightness would be preferred. There's always the PipBoy light, but I hate that thing (makes it hard to sneak, too).