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• Temperature. I know that Disney's done some things like have water jets that spray at people for their rides. I imagine that someone could rig something up with a computer-controlled heater/air conditioner, make a cave in-game feel cool and clammy. Kind of hard to both set up in arbitrary places and get a common experience across people. There have been people playing with using thermocouples for VR.

• Scent. Again, I recall reading that Disney worked on incorporating this into Fantasia presentations many decades back. I suppose that one could have some kind of device that kept a bank of essential oils and then could mist a given set, though I think that how long it lingered would depend a lot on the surrounding environment. Maybe if you had some kind of vent by the nose or something that passed air directly by the face. There are apparently companies trying to sell devices to do this sort of thing.

• Motion simulators exist, can be used for stuff like flight sims, but I think that the price point is probably going to remain too high for them to really take off for the mass market.

I mean, in theory, I could imagine someone writing some kind of software layer that cleverly abstracts all these sorts of systems, has games drive events. But in practice, I've been watching external devices that aim to provide a new source of sensory input come out for a long time, and they have pretty depressingly and consistently failed to take over the market.

I think that realistically, in terms of having many games providing sensory input to people, it's gonna be the display and audio, and maybe VR headsets for the foreseeable future.