There are some definite downsides to day/night systems. But I think the downsides can be handled in intelligent ways, and it's worth it for the extra immersion it adds to the world to have things be different at night. It always seems a little fake if everything is always exactly the same.

 

Of course, day/night dependent access to shops, quest, monsters, etc., requires some form of waiting system, but that's something dozens of RPGs have managed just fine.

 

It's also cool when NPCs move around as long as there is some sort of tracking system to find them. I get it's more realistic, but I lost track of how many hours I spent running around the Mage's College in Winterhold in Skyrim trying to find the various #$%&ing mages. And that's not even a large college....

It may be interesting to see what sort of day/night cyle the world has - how many suns or moons - what sort of sky coloring & clouds and general weather.

 

As a design choice, would clouds float across the screen, limiting your vision of them or should you see shadows of clouds on the ground?

 

Torches during the night?

Is vision going to be a mechanic? (brighter = easier to hit, darker = harder to hit, just as you can't see as far during night as you can during day... unless having infravision or something)

How dark can dark night get? So dark that you have to have a Torch or a Gamma Option? Could there be a "Darkness Options" altogether where the Torch still lights up (even if Low Gamma) just as much as it would light up with High Gamma (basically not make the torch-lighting be affected by Gamma).

 

If you are out in the middle of a forest without any light, isn't it almost pretty much pitch black?

In dark conditions with no light source, it is possible for your vision to adjust to night sight over the course of a half hour so that you can see a surprising amount of detail. This is known by astronomy hobbyists who try to maintain dark conditions so they don't have to spend another half hour getting their full night sight back.

 

Another factor in this equation is the type of moon system found around this planet. It may not necessarily be the same as the Earth. There could be multiple moons, a ring system, or the world may be a moon orbiting a giant planet. Any of these factors could cause the night time to be brighter than on Earth.