> do you think it's possible as CTRL-labs propose that we could entirely rewire our nervous systems' output to let us operate computers as if we had multiple hands?
Yes. You would not quite do it that way though. Your motor cortex is capable of handling that sort of thing[1] but your conscious awareness isn't[2] so you have to develop a kind of expanded awareness, like a dancer or martial artist, but in an abstract space.[3] (I suppose you could just hallucinate having eight arms, now that I think about it.)
You don't have to wait, you can get results with crude hardware. The trick is to use hypnosis. See my other comment in this page[4].
We could have fantastic "magical" UI a minute ago, but people connect a brain to a computer and then focus on programming the computer. It's silly.
(BTW, our output channels are primarily the face and the whole body[5], but there's evidence that sign language predates spoken language. I've heard that pre-verbal babies can learn to communicate with gestures.)
[1] Higher dimensionality or more degrees of freedom
[2] Generally, for most people. However it is possible to train your mind to carry out two or more simultaneous activities, such as writing a sentence with one hand while solving a math equation with the other. But this is sort of a stunt. Most folks would never conceivably need to do something like that.
[3] Obligatory reference to "Cyberspace", and "Minority Report" UI.
[4] https://news.ycombinator.com/item?id=18564989
[5] Gait, posture, overall tension/ease, etc.