Bowed Marimba and Vibraphone?

I hope Pianoteq develops a variety of rich sounding strings that can be cross velocity faded in a layer with with the piano.

Bowed marimba is fairly esoteric.  You could also request the running of your finger nail down the wound bass string of a piano with the damper off, but there's not a huge call for that.  Bowed cymbal and gong can be a nice effect too.  Perhaps a "prepared piano" instrument could be developed.

There could be a Victor Borge piano with the sound of the player falling off the bench or accidentally slamming his or her hand in the key cover lid or music rack.

There could be the Piano Tuner model, where the intervals if single strings are brought into adjustment  such as 5ths, 4ths, octaves, major thirds, minor thirds, 10ths and so forth with the coups de grace being the tuning of unisons.  This would include the sound of a heat gun melting the glue holding the hammer heads.  Let's not forget the sound of driving larger tuning pins into the pin block, this would include the sound of all the undampened strings sounding in sympathy to the blows of a hammer.

Finally there could be the Tibetan Singing Bowl instrument which would include 88 pitched Singing Bowls.   The bass instruments would be completely fantastic.