First thing that comes to mind for me, is being able to properly smoke cigarettes, cigars, and tobacco pipes as well. It's such a small detail, but the Rockstar design of smoking a single puff of a cigarette isn't realistic at all. It would be cool to see that you can light a cigarette and then either puff away on it as you wish, or just keep it in your lips between puffs. Maybe 5 minutes or so would be much more realistic, with the option of putting it out earlier if you choose. I loved this mod by HUNK for GTA V:
https://www.gta5-mods.com/scripts/bad-habits
Maybe he or somebody else will be interested in doing something similar for RDR2