I think a more historical period clavichord, and an early virginal, would be welcome additions. Pianoteq has shown with their H. Ruckers II Harpsichord that they can delve into and reproduce the nuances of early instruments in great detail.
I would imagine that they chose the Neupert clavicord to model because it's a practical "compromise" instrument, especially since the Neupert has a range that is more than an octave wider than that of a historical 18th-century clavichord. The other factor is that the Neupert is "unfretted". I would imagine that taking double or triple fretted designs into account would enormously complicate the process of modeling the instrument.
As to you second suggestion, the Pianoteq program is never going to provide an organ sound. What Pianoteq does is model the sounds of struck or plucked strings (piano, harpsichord, concert harp) or struck metal reeds (Rhodes, Wurlitzer, Hohner electric pianos) or tuned percussion (bells, marimba, xylophone), and it does it all through physical modeling synthesis. A portatif organ would require physical modeling of organ pipes, or digital samples of organ pipes, and that is totally outside the scope of what the Modartt company creates and develops. I suppose Modartt could choose to offer an organ product at some point, but if they did, it would certainly be in a totally separate app from Pianoteq.
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