I think there are several lessons that can be learnt from RDR2:
1) Animations: number and quality. That's how an immersive game should be, everything is so realistic (with some exceptions). It has animation for eating, picking items up, opening doors, smoking... When playing in FPP it really feels like you are doing those actions, you never feel like a floating camera moving object via telekinesis. Extremely immersive. Seen in other games, but never at this quality.
2) Polishing: the game has very little bugs, almost no need to be patched after launch. Incredible, every software house should learn from it.
3) How narrative is perfectly coherent with side quests: there's a perfect combination of moments when the game pushes you towards the main quest (prologue), moments when it gives you time to explore and do side quests ('till Saint Denis) and again and again. You feel in a hurry when you should and you feel like you have time when you should. First time I see something like this in a videogame, in particular RPGs have this probelm usually.
4) world changes with time, according to narrative. I don't want to spoil too much, but you can see houses being built with time and . That's an incredible level of details that pairs with animations, with getting dirty with blood and when falling in mud or getting washed when raining or swimming.