I agree with Prospero that for a soloist, a low-key encore makes sense. A slightly different question is, What happened to orchestral encores at the end of the concert? Henry Fogel has lamented their absence, noting that in the past, it was a chance to play a bonbon, which we might consider analogous to a frothy dessert. Perhaps not the best idea after M6, or any other Mahler symphony.
End-of-concert orchestral encores are largely extinct at subscription concerts, though sometimes visiting orchestras will play them.