As I see it, it seems Larian tied the passage of time to resting, as a way to introduce a kind of time management without having a real time-based time limit to quests and, hopefully, main story related events. I think a classic day to night cycle wouldn't work too well for the game then. If they were to implement Day and Night, it should be tied to resting. There should be a day and night version of each map with important differences, and long rest would change from one to the other. Night maps of course have their own encounters and change some existing ones. There could be wolfs at night and maybe the goblins in the goblin camp would be sleeping, but some would be patrolling with torches to find you if you are hiding in the shadows. And stealth would be very different thanks to the shadow and light system. The problem is that it probably would be a lot of work, specially if they want to keep reactivity, but could be feasible for the definitive edition. They did remade Act 3 of D:OS 2, so big changes aren't out of the question, but these may be just too much.
I don't know exactly how dramatic the effect of resting is on quests. There also should be important consequences to resting too much through the game. Similar on how Pathfinder:KM makes some event and quest change or fail if you take too long, BG3 should encourage as few long rests as possible to get the best results in all quests. Also Larian should have the balls of locking you into a bad ending if you rest too many times during the game. You should reach the last chapter and the tadpole should kill you if you "took too long".