Even a simple day-night cycle would be a huge increase to immersion, and it really wouldn't be that hard to implement it.
Simplest day-night cycle: either based on an in-game clock or after X short rests, the game turns to 'night'. No fancy transition is needed. Simply the next time you exit a loading screen, it becomes night. NPCs are either still present or missing (sleeping), and potentially Larian could add new NPCs/monsters. When you long rest, it becomes the morning of the next day.
Quests should only care about whether it's night or day, and possibly the # of days. E.g., "meet me during the day or during the night" and possibly the rare "quest fails if you rest after beginning it: e.g., Waukeen's Rest burns down." Time-limited quests should be very rare and explicitly marked as such.
No matter what Larian chooses, the journal's timer should be changed. Currently, I believe it only updates when you have conversations/encounters..? It should certainly update when you long rest.
