There is a dearth of reparatory for solo viola and orchestra. I think that my fevered mind came up with a good one. I see no compelling reason why Rachmaninoff’s Rhapsody on a Theme of Paganini (Op. 43) could not be transcribed for solo viola and orchestra. Of course, once that was done very little effort would be needed to adjust the scale for the needs of the cello player.
The orchestra sheet music has lapsed copywrite and is widely available. The solo transcription could, I believe, be protected by copywrite to generate some fees for the author. I can see ample demand on the various Pop’s festivals once covid restrictions are behind us.
Success is the progressive realisation of a worthy ideal. —Earl Nightingale.
This might help - most of it's in Alto Clef, even though it's a Cello solo with Piano.
...your idea of fun? 🤣