So, I was thinking... What if the air had modifiers?
I suppose the smell mechanic would already function like that, but it can be made more significant, giving you an immediate reason to pay it heed rather than just when hunting for a specific plant, or when you think you're being trailed. Right now (as I understand it, again), smell is nonessential, it's usable to track objects, animals, and people in some capacity, but that's about it. While it can communicate immediate danger sometimes, what if we allowed the smell itself to be the immediate danger?
For instance, miasma could be one of the air states, and would introduce a higher chance for infection or plague as well as smelling terrible.
More interestingly, you could expand this concept to restrict the spread of certain plants and animals and create codependency. For instance, Tembra(the black mold I discussed that would destroy crops, claim houses, etc) might develop large sporangiums that burst after a while and dump out groups of mobile slime or perhaps imp-like creatures that are dependent on the toxic air surrounding the mold to survive.
In caves, if you don't create an air source using subterranean plants, you might run out of air and begin suffocating, slowly but surely. Perhaps you end up hitting a pocket of sulphuric air due to a nearby volcanic vent that inhibits your vision as well as being toxic. Maybe you hit a flammable air pocket, that will ignite and cause the entire mine to go up in flames from an errant spark while mining if you don't let it air out first. There could also be a sadder aspect of this. Perhaps some subterranean creatures dependent on a certain type of air can be uncovered, but at the cost of causing their air to be siphoned off in exchange for oxygen and slowly killing them if you don't do anything. This would introduce a particular challenge, too. Subterranean communities and homes deep underground would need to develop sophisticated ventilation systems. Perhaps certain infections, plagues, or symbiotes could switch around the type of air you can breathe, too, making you entirely dependent on a certain air type if left untreated.