Some instrumentalists generously include members of the orchestra in their showpiece. Simon Trypceski became accompanist in his encore by inviting London Symphony Orchestra leader Roman Simovic to perform the final section of Ravel's Tzigane and Janine Jansen has performed Bartok duets with the leader on more than one occasion. In these situations the concertmaster obviously has to agree to this, usually at extremely short notice!
Another violinist Vadim Repin would return to the stage, turn to show the violins a simple pizzicato riff to be played a la guitar and then perform variations by Paganini on a carnival of Venice. A fun piece which shows off the violinist's virtuosity which the audience love.
However, of the numerous violin soloists I have had the honour to accompany, the vast majority perform the Bach Sarabande in D minor. Now I don't have anything against Bach and I guess it's soothing to have something contemplative after a huge work like the Brahms or Shostakovitch concertos but I can't help but feel it would be more exciting to present a more original encore. Granted there is not a huge amount for violin solo. The Ysaye Ballade sometimes presents itself, although it's a touch too long for an encore in my eyes.
We've had pianists such as Yuja Wang and Denis Matsuev perform incredible arrangements, the piano obviously lending itself well to such things. But still for the violinists I would love it if a soloist had a specific piece and made it their own. Surely they have time to prepare a 3-5 minute piece to leave the audience gasping for more. The Greek violinist Kavakos had members of the orchestra agog the first time he performed the famous Recuerdos de la Alhambra post-concerto, not an easy feat transferring the famous guitar solo to the violin but he did it with effortless ease and virtuosity.
I'd like to think a soloist might commission a short piece for solo instrument to present as an encore. What a great way to showcase a young composer and have something really original to play. So any composers out there, write some short pieces for solo instruments and send them along to soloists, you never know, you might find yourself being premiered before you know it!