ampfixer
Tuned tupperware tubs?
I think it's been done, actually.
In fact, I really can't think of any useful sound-producing object that hasn't already been sampled.
As to what Greg's got up his sleeve, I'm going to guess a melodic instrument (as opposed to percussion, found sounds or nature sounds), since that's always been his focus. Guitars he's already got covered, certainly basses. The flute was a surprise, and well-received, so it's possible he might go further in that direction, perhaps a saxophone. The world could still use another sax, IMO. But if I had to place a bet, it would be something more difficult and further afield from his usual endeavors. I'm gonna guess human voice.