If you keep up with the whole trans-humanist movement, then you know all about the predictions made about how humanity is about to drastically alter itself with technology. Even if you don't buy the more out there claims made by futurists like Ray Kurzweil, it is generally recognized even by skeptics that we will be doing quite a bit of tinkering in the brain and cognition with technology over the course of this century. Check out (http://en.wikipedia.org/wiki/Transhumanism or http://humanityplus.org/ or http://www.nickbostrom.com/ethics/values.html).
Compared to the kinds of tinkering to come one hears tell about, a relatively simple idea would be 3D voxel retina. Here voxel stands for volumetric pixel, and pixels are analogous to the photo-receptive cells on our natural, 2D retinas. Whereas with our 2D retinas we experience 2D color data sets and perceive 3rd dimensional relationships, such that we feel immersed in a 3D world rather than a 2D world, likewise, with a 3D volumetric retina we would experience 3D color information sets and perceive 4th dimensional relationships, such that we would, in time, become as accustomed to the 4th dimension as we are to the 3rd dimension.
Of course there would be no real way of gathering 3D data from the real world, since light must hit a surface, not a volume. But software designed to stimulate virtual nerves arranged in the form of a cube or sphere rather than a square or circle, could expose users first to basic hyper-dimensional shapes, like the 4D analogies of the platonics, allowing users to slowly come to terms with what they are seeing, like a 2D being experiencing 3 dimensions of color information for the first time.
This would be a very useful technology, as it could probably improve intelligence. As users are trained in understanding what they see in their 4D perceptions, they will gain skills in reasoning in 3 and even 4 dimensions, rather than the 1 or two that we do. For instance, if you are planning a complex algorithm, you can only visualize it or think about through 2D diagrams, otherwise there is a lot of crossover that makes things confusing.
Even though you would not be able to perceive real-world data, after plenty of practice understanding 4d geometry and 4d functions, the brain would adapt to this data and you would develop or experience immediately an entirely unique color experience, which you could continue to experience with or without the implant, as your brain would have adapted to handle and process 4 dimensional data.