I have so far not heard a convincing piano synthesizer, i.e. one that would fool a pianist.

Pianos are actually really easy, input-wise. You have attack speed, when you lift the keys and pedals, and that's it. But simulating the audio computations done by a physical instrument is just really difficult.

There are some physically-based (spherical cow physical) synths that sound more alive than the usual sample-based stuff, but their timbre is not right.

If I had more time, this is what I'd be working on right now. There's a market for a great piano synthesizer.