What is a sense? In the most abstract interpretation of the word, it's a stream of information about your surroundings, which arrives in your mind without conscious effort. People usually say we have five of them, but this interpretation is actually broad enough to give us a few more. For example, we have a natural empathic sense, in that we can sense the moods of people around us, based on how they act. This happens without conscious effort, and gives us useful information about our surroundings, so we may as well treat it as a sense.
However, the fact is that there is a lot of stuff that goes on in the world that we're totally incapable of directly sensing. This really should bother us more than it does, but nobody really notices, because we can't see, hear, smell, taste, or feel these things. Out of sight, out of mind, or something like that.
I'm not content with that. It bothers me that there's a whole world of stuff that I'll never see. Luckily, I don't have to just live with it; we live in an age when most things are possible, after all. If this kind of limitation is part of being human, then fine, we can become more than human. (Some people claim that this sort of thing will eventually cause us to lose our individuality and creativity, for the sake of technology. Some people watch too much Star Trek.)
The first goal is the electromagnetic (henceforth, the EM) spectrum. We can see a relatively narrow slice of it - the human eye picks up the bits that happen to have wavelengths between 380 and 760 nm. Even just extending that a little in each direction, to encompass infrared and ultraviolet, we find enormous possibilities. Imagine, for instance, a firefighter that could see in infrared, and track the spread of a fire on the other side of a wall. Going up past UV in the spectrum wouldn't be quite so useful, simply because there isn't that much high-energy radiation flying around. Nuclear explosions would probably look pretty flashy if you could see X-rays, but then, they're already kind of hard to miss.
Q: What would happen if we could suddenly see through walls?
A: We would start building better walls.
Another neat thing that came up recently - the haptic compass. The idea here is to basically give you an intuitive sense of direction. People who have tried this report a huge improvement in their ability to navigate through cities, proving that the brain is perfectly capable of picking up a new sense quickly.
But this is just the beginning of what's possible. The EM spectrum is what I'll call a "direct" sense - if you were to see it, you'd be seeing actual photons that are zooming around. It's directly tied to a physical phenomenon. It's possible to imagine "secondary" senses, like my original example of the empathic sense, which are aggregations of data from the other senses.
In a few decades, a set technologies which are only just getting accepted into the mainstream now will converge in such a way as to create something completely new. Ubiquitous Internet access, wearable computing, and some as yet hypothetical advances in computer vision will allow us to get information instantly about anything we can look at. It's not a huge step from there to imagining a system which tracks what you're looking at, and shows you extra information about it. If done seamlessly enough, I believe that this could be natural enough to call a new "sense".
(As I am writing this, a news story has just come through my RSS reader - a group at MIT has done something similar. It's obviously a pretty limited version of what I'm describing, but since it's MIT we're talking about here, they're probably thinking along the same lines as me. Awesome. :3)
But why stop there? Let's get into real sci-fi territory. In forty or fifty years, it's not inconceivable that we'll have direct interfaces into our own brains, allowing for the creation of new senses on the fly. The brain is extremely good at aggregating large amounts of raw data and making sense of it, so anything which generates a lot of hard-to-visualize data would be a good candidate for this.
I'm sick today, unfortunately, so I'm ending this post here. :(