Not sure if I understand the question. What non-functional things did Stradivari add? He made good fiddles, but by the time Stradivari was making instruments, most of the basic violin design - and the basic styling- had been around for quite some time, with carved top and back, scroll peghead, purfling, deeply inward- bent side bouts. Andreas Amati had put a fourth string on it, by 1564, but even before him there would have been something recognizably violin-like. There's some history and good iconography of medieval fiddles here