I want them to model more harpsichords. I find the current set to not really quite hit that clean, crisp sound of a super nice one yet:
https://www.youtube.com/watch?v=JXi79RGTXiA
Also, some higher quality pianofortes:
https://www.youtube.com/watch?v=Ua_GHVEDCdI
Bosendorfer, as well:
https://www.youtube.com/watch?v=bWZR4fzc590
I'd also specifically look for some touching up on some of the existing pianos, adding to the base sympathetic resonance without getting too many high pitch/buzzing overtones when you increase it. That's one of the big things that I notice about a recorded piano vs. pianoteq, and that's a difference in the resonance in the wooden tones of the piano. I.e. this is a good example:
https://www.youtube.com/watch?v=mfprFtmTb5k
I.e. here I can pick out a lot of very high overtones past the 8th. You might have to close your eyes a bit, but it's clearly the overtones and resonance of the cabinetry of the piano itself reverberating throughout the room. In addition, the reverberation is noticeably cleaner. That room has great resonance, but there's an unmistakable sound to a live piano, as well as a recording that I feel isn't exactly captured yet, but Pianoteq is the best I've found at capturing the sound. So, I'm investing some time trying to point out more constructive bits that are missing.
Basically, I hope soon that Pianoteq is indistinguishable from a real recorded piano with the highest fidelity, though right now it's the closest and most people probably can't tell the difference.