Even though dusk and dawn would be amazingly immersive, I would settle for a toggle type day / night change.
And I'm not expecting NPC's to all act differently after nightfall. I'd rather have night time where NPC's "stay up really late" or just mysteriously disappear into their homes or wherever, than no night time at all. The atmosphere is that important. It feels completely off having to do sneaky thievery stuff in broad daylight. Mechanically darkness should also grant stealth advantage.
