Nighttime ideas: (In addition to the normal nighttime mobs: Bats, Bogy, Skeletons, Zombies, etc.)
I) During a New Moon and/or Umbral weather, you might encounter ultra rare lvl 100 Ascian HNMs (not really killable). It should just fade in, kill any players they encounter and fade away. It also should be able to pass through terrain as well. People would freak... :D
http://i.imgur.com/b2kJ8.png
III) Doomgaze flying through the sky at night in a certain zone?
V) Parties of bandits that randomly appear on roads away from towns/cities and attack players as they pass by (give them ranged weapons or spells to quickly dismount Players).
VI) Ghost Ship that travels the ocean at night. It could attack ferries in the same way that the one in FFXI did.
IX) Mobs that change slightly at night: Glowing eyes on certain Rats and Wolves , Goobbue with Bioluminescent plants on their backs. Firefly swarms that light up at night, etc..