I don’t know much about the technicalities regarding musical instruments but it seems that lots of them, from organs to harps, are rather sophisticated and designed for a specific purpose. This leads me to believe there is room for more but I haven’t heard of much innovation on the traditional instrument front.
Is someone working on something equivalent to the piano for instance?
Well, there are the instruments created by the Blue Man Group:
http://www.blueman.com/music/instruments.shtml
The Stick is the most recently invented musical instrument I can think of that’s actually manufactured in quantity (as opposed to improvised, one-off inventions like BMG uses).
Yes, of course, but that you haven’t heard of it makes perfect sense. Not many folks are creating new physical, acoustic instruments… nearly everything’s gone electronic. Sure folks are trying to top the piano, but I doubt someone’s had any success in the physical realm. The terms of your Q exclude 99% of the innovation that’s currently going on, because like it or not, it’s where the money is.
Too bad Mozart didn’t have one of these. He would have finally achieved the level of creativity that otherwise eluded him.
I often get the feeling that not many people are making mechanical anything these days. Everything is electronic, even things which work quite well in the manual version, people are trying to fancy them up one way or another.
For example, today’s home page links to an article by Unca Cece about how the manual “dim” setting on automobile rear-view mirrors works, and he tells of some Cadillacs now (1989!) can do this electronically.
There’s the daxophone. The inventor’s web page is one of the most creative and amusing websites I’ve ever had the pleasure of exploring. (And that’s high praise coming from me, who normally can’t abide Flash-based websites.) The web page makes extensive use of daxophone music and meerkats. 
The Chapman Stick is an electric instrument, though, whereas the OP was asking about purely mechanical ones.
All the time here in newfoundland, My teacher, Rob Power makes new instruments all the time. One of them his “Quarter tone Mirrorphone” (a instrument tuned in quarter tones, with the bars made of broken mirror" was recently featured in a new concerto for percussion and string orchestra.
Boomwhackers postdate the Chapman Stick, and are probably made in a far greater quantity ( != quality, though!). Of course, neither are an ‘invention’, and nor are 99.9% of instruments of any age. They’re refinements, adaptations or elaborations (or even simplifications) of previous stages of development.
The OP asked about non-electronic ones. The Stick is an amplified non-electronic instrument. Another variation on the guitar is the Novax Fantail, with non-parallel frets. But it’s more a variation/extension than a new instrument.
I hadn’t heard of the Chapman Stick, so I decided to see what it sounded like
I have no basis for comparison, but this guy is incredible, IMO.