I don't think it's as much of a technical issue as it is a practicality issue. You could probably keep 50-100 small vials of aromatic chemicals in a reasonably small area, and then you could just have an SDK that allows the programmer to determine the amounts that each chemical gets released with triggers in the game. The problem is it requires game devs to hire frangrance chemists to be able to know what to combine to synthesize a given scent, and it requires the player to either insert something in their nose or stay close to a frangrance sprayer for it to work. With many people already being hesitant to use HMDs because they don't look cool, I don't see this as an easy sell. Also I'm not sure you want to give game devs that much control over how your room smells.
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