I'd like to see more variation between day and night too. As well as having nocturnal mobs such as bats / undead appearing at night, other mobs could exclusive to day time or sleep during the night, I remember some sheep in XI that did that.

I also think it could be a good mechanic to change safe paths through zones depending on the time of day, so at night, aggressive mobs come out blocking some routes but daytime mobs blocking paths sleep allowing safer access via a different path. Adds a bit more variation in travelling to different places. Obviously this could be tricky with most of the current maps, but it could be something to bear in mind.