Just an interesting though here. If you could take any concerto already composed, and have the composer replace the solo instrument(s), what would it be? It would obviously involve some alterations to the score to fix some conflicting issues, but if the basic themes and harmony were the same, which pieces would you choose? Make sure it is based on a concerto already composed by that composer. In other words, you could say a Beethoven Cello concerto made from the Violin, but just a new Beethoven Cello concerto wouldn't work.