There are a few ways this could be implemented:
-Nocturnal creatures: irl, night brings out some dangerous predators. Maybe in Skyrim, giant spiders venture out of the caves at night, or maybe wolves are more prone to hunt.
-Nocturnal NPCs: I'd like to see werewolves and vampires as extremely dangerous foes, coming out only at night. If you travel during the day, you know you're safe, but if you get caught in the wilderness at night, you run a serious risk of getting attacked.
-Fast travel: In order to make getting caught in the wilderness at night feel like an actual danger, you shouldn't be able to click on a magic map and safely transport to a town or inn. It'd be interesting if you had to plan your travels to avoid getting caught in the wilderness at night or run the risk of facing increased danger.
-Criminals: This could be implemented not just in the wilderness, but also in the towns and cities. NIghttime should increase the chances of falling victim to muggings and the like.
-Darkness: If you've ever been out in a forest at night, far away from city light, you know that it can get pitch black: so dark that you'd run into trees without a light source. I think on certain nights, perhaps when the moon isn't out or there's a lot of cloud cover, you should need a torch to see where you're going. Imagine walking through a forest in the dark and being able to hear a creature follow you through the trees, but not be able to see it until it pounces It could add a ton of excitement to otherwise uneventful travel.
What do you think?