RE: day/night cycle
A day/night cycle is like one of the easiest things to pull off, you just need to know how colors work; it's literally just a dark blue-violet glaze on top. And it's the same both for 2d and 3d, although I guess it would be more complicated with modern graphics and many sources of light. But really, I made this in 5 minutes:
1. Random BG internet picture:
2. My own lazy, cheap night version:
3. How it's done:
Same with BG3:
And if you want this to affect the shadow mechanics in some way, just add a multiplier to whatever bonus you got from the daylight shadow, like 1.5, and that's it.