Orgrinder,
Well, if I'm going to dream, I might as well dream big!
I would have loved to play the organ part in a live performance of Mahler's Symphony No. 8, with Leonard Bernstein conducting. I've been lucky enough to hear it performed live twice, both times with the New York Philharmonic, once with Pierre Boulez conducting, the other with James Levine. I get chills just thinking about the experience! To participate in a performance of that incomparable masterwork would be a thrill.
And, in a different realm, it would have been a dream come true to have played the Hammond B3 with Jimmy Smith, accompanied by a screaming big band, with Claus Ogerman or Lalo Schifrin conducting one of their great jazz arrangements! We'd jam until the Leslies melted.