Yamaha makes this ultra-fancy piano that consists of a real grand piano with an additional system for detecting keypresses (i think it optically measures the speed of the hammers) and electronically triggering the hammers, so that you can record and replay performances. You can also make the piano physically silent but trigger a synthesizer using the detected keypresses so you can practice quietly with headphones. Replaying a performance makes the hammers actually hit the strings again so you get the real effect of the performance as played on the actual piano. These pianos come with floppy disk drives for storing recorded performances.
Now, if they can do that (and these pianos have been out for several years), why can't they add a nice high-resolution LCD to the piano for displaying the sheet music? Pop in a CD with the score you want to play and touch a pedal with your left foot to turn the pages. Or, include a score in a data track on your CD of Tchaikovsky's piano concertos so that you can play along to the symphony orchestra.
As long as we're dreaming of the ultimate piano, let's have the LCD be a touch-sensitive tablet as well so you can mark up the pages while you're practising. And what if the piano had a network port? You could broadcast a performance played in one place to multiple pianos over the Internet, or play duets with a friend who has a similar piano on the other side of the country. There are a couple of ways you could do the duet: either your friend's playing would be transmitted as audio where it would mix with the real sounds coming from your piano, or your friend's keystrokes would be transmitted as data so that your piano would be playing for both of you. Imagine jamming on the Internet with a whole bunch of other people all playing their pianos together — you pick the ones you want to hear, and broadcast your keystrokes for anyone else to tune in.