Okay, so it's fairly simple harmonically. I wouldn't mind seeing some chordal notes that aren't 1-3-5-7. But that might not be what you are into.Those fast harmonics might be a little hard to pull off, maybe? I'm not sure. Have you had a violinist look at this yet? Probably not, now that I think about it, since you just wrote it. EDIT: I see now that you are a violinist, and I a fool. Nonetheless I will keep my comments, so maybe you can better clarify the limits of string instruments for me.
Variation #2 reminds me of Dance of the Jesters, with that god-awful run that I had to play on xylo once. It was fun, just...ridiculous
So variation #3...that seems hard for the violin, but doable I think. I think the biggest problem would be moving to different strings?
Variation 4 is cool, not sure about those octaves though...or the double stops in 132
Same thing with 147-153. If the repeated note was on an open string, that might be easier. I don't know, I'm not a violinist, and I feel a bit nitpicky about the violin part. I think I definitely write more conservatively for strings that I have to, so this might all work, I don't know.
Thanks,
JWhitmarsh