Seeing and hearing is what games have always used: you can see things happening on your screen, you can hear the music and sound, but what if we were to get smell in games? I remember reading an article in a science magazine a pretty long time ago where a company tried to create a "box" that would send out chemicals that smelled a certain way when we did things in a game, but i guess they failed because im sure we would've heard of it by now.
I could use a game like BF3 as an example: You can smell the gunpowder, the diesel from tanks, the sweat when you run for a long period, etc etc.
Im not saying it would necessarily be a good idea, but it could be a way to add immersion. What do you think?