I'm kind of disappointed about opting not to do the day and night cycles, because:
1) You're going to need to code the ability to change the sky anyway when it turns red at the end of the game.
2) realMyst did it. Part of the whole reason I want Starry Expanse is to see what Riven would look like if it were a living, breathing world rather than still images. What's the point of making the game in realtime if you're not going to take advantage of all the things realtime allows you to do? You might as well just have nodes you stand in and click to move forward and keep everything still then, and just have Riven exactly the way it is with updated graphics.
3) It is canon that Riven has a moon because it is the first thing you see in the intro to Myst, so it'd be good to have a nighttime so the moon can be seen, and the night was gorgeous in the Unity demo from 2012.
For the amount of effort it would take, I think adding a day/night cycle would make Riven even more real. I realize that I'm not the only opinion here though, and that if it wasn't done well a night cycle could make it difficult to see. But cmon, you're telling me you can 3D model the entirety of Riven, bending water with code... but can't add a day/night cycle, a common thing that exists in a lot of other games already and as such could probably be implemented with ease?
I have to agree about weather though, having rain would mess everything up. How would it interact with lava, interact with other water, would it make ripples in water which is being moved over because of heat?