If I were to somehow magically improve one skill that all my students need to work on then it would definitely be their vibrato technique. Vibrato is an essential technique of expression and can take a simple boring melody line and make it sound amazing when done right. Here's how to go about practicing it.
Supporting Lessons;
4 Note Improvisation: https://www.sheffieldguitarstudio.com/post/improve-your-improvisation-with-4-notes