The ability to assign vibrato [designated by pitch width, frequency, velocity and speed] would add a rich dimension to strings. I see this as similar to the trill. The sign is the wiggly line. String players differentiate between the fast vibrato [narrow wiggly line] and slow vibrato [wide wiggly line] - these options would find a home.
Multiple trills in Sibelius 7 sounds results in garbage on playback. The trills do not terminate after the note ends and make the following note or notes trill. After two or three trills the trill then continues in all following notes for about 4 bars.
Today, an accidental added to a trill is a symbol and just a picture. Desired behavior is that If I change a trill to be non-diatonic in the inspector, a sharp, flat or natural appears automatically with my trill in my score. Conversely, adding an accidental to the trill adjusts the playback to match.