My rum-fueled retort...
One thing that remains to be done with electronic instruments, in my opinion, is to create an instrument which is as expressive and controllable as an electric guitar.
I can play electric guitar to a certain extent, and I can play keyboard to a certain extent. The thing that has always struck me about keyboard instruments, (even a real piano) is the very limited control you have over each note compared to the control and latitude you have with an electric guitar.
With a keyboard, for any given note, you typically can only control how hard you strike the key, and how long you hold it down. That's it. Sometimes you have wheels, or ribbon controllers, but these typically require huge gross crude movements of your entire arm to operate, whereas, the effect you might be going for could be effected on guitar with the tiniest of finger motions. All the keyboard samples of electric (or acoustic) guitar are pretty lame, precisely because you cannot control them as precisely as you could with a real guitar. The sounds themselves, taken in isolation, are fine. It's when you try to perform with them that the limitations become apparent. They end up sounding canned, because they are canned.
My point is, the electronic equivalent of the electric guitar remains to be created. I'm not knocking either electronic music or electrontc instruments, just saying, "here's something that hasn't been done."
I'm making the assumption here that the electric guitar does not count as an electronic instrument, even though electrons and electronics are involved, since the instrument called "electric guitar" consists of a guitar and a guitar amp. (The amp is definitely part of the instrument, contributing much more to the sound than just amplification.)