There are millions of receptors that inform the brain about what the body is currently doing, which muscles are shortened and which are not, how the gravitational situation is like and so on. A feedback loop tries to reinsert this kind of information into a BCI.

The eyes offer a simple and efficient way to give the brain feedback without such an artificial loop, but for complex tasks this is probably not enough. Think of someone with a BCI-controlled prosthesis who would want to use it at night, for example. If it is sufficiently dark he won't see his leg, so the visual feedback way is blocked. Being healthy, he could rely on his sensory system, but a simple prosthesis won't have one, at least at the moment.

So the only chance are different, artificial modes of feedback. This is definitely a complex task, as it involves feeding information back into the brain. For such a BCI, the brain would not only "speak" a new language (EEG or firing rates of neurons), but also receive messages in yet another unknown language. But do not worry too much: All this might work, as nerve cells seem to be far more adaptive than previously thought.