The sound of digital pianos continues to improve, and expensive ones also have a good feel, often by building individually weighted keys that go beyond simulating a key on a real piano.
What might be done with more modern technologies, such as super-fast servos, and fluids whose viscoscity can be varied based on the strength of electric or magnetic fields applied to them. (Some of these fluids are being applied to the development of dynamicly responding shock absorbers.)
So the first step would be to build an action to connect to a keyboard, be it either a servo, a fluid or just a plain powerful magnetic coil, so we can adust, with millisecond resolution, how much backwards force the key applies to the finger of the player. Of course we must also accurately and quickly measure the force being applied by the finger to drive the process.
Next, we would build a device to measure the force-response of a real piano keyboard. It would press the keys in various ways that real players press them -- slowly, quickly, hard, soft and with other forms of varying touch measured from real pianists. Then attempt to develop a model of how the keys on the real piano respond.
With this, we could measure all sorts of great pianos. The concert Steinways, the finest pianos available. These all feel different. In some cases the feel is not necessarily "superior" but just what people have come to expect from that type of piano.
Then we would program our dynamic resistence keys to model any piano that had been measured. Throw a switch and change how it feels from Steinway to Yamaha. Just as you can throw a switch to change how it sounds. Ideally, the equipment would be light so the keyboard would not have to be heavy, as today's weighted MIDI keyboards are. (Of course they are still much lighter than grand pianos.)