How To Write A Showpiece

A good friend of mine is a violinist and I have decided to write him some sort of showpiece for his birthday late in January. I figured this wouldn't be too hard to pull off, but I've been having trouble getting started on the project. I myself am a proficient violist (working on the second movement of the Walton), but, of course, violists aren't generally the same kind of virtuoso speed demons that college-age violinists are. Anyway, I am wondering how composers write virtuoso showpieces for instruments they don't play. I assume it takes some level of collaboration with the artist, but from where does one draw inspiration for such a piece? Should I just start writing, then add all the virtuosic fluff on top?

Here are some of the criteria I have set for myself:

-very showy but with some level of musical depth (so it sounds like it was written by a good composer, not Paganini)

-violin with piano accompaniment

-utilizes a good number of virtuosic violin techniques (left hand pizzicato, ricochet, some tricky double stops, etc.)

-incorporates some of my friends suggestions/requests (he likes Paganini, Saint-Saens, Shostakovich, and blue-grass, fiddling, and jigs)

Any ideas or suggestions are much appreciated.