Wow! Great sketches. Your enegy, dynamism, and expression are fantastic, each sketch is clear and immeidately invokes an emotion. I especially love that you don't shy away from mixing lines (a graphic approach) with values (a painterly approach) and indicate shading, even if it's just two tone.
If you want to challenge yourself, I suggest changing your style to a more realistic one. Currently, it's more cartoonish (the size of the eyes, the exaggeration of the eyebrows, and the stylization of the teeth into blocks). As you change your style out of your comfort zone, I think it'll help you in your future sketches.
Another nitpick is the blue man's back, I find the muscles hard to read, perhaps due to the lack of a spine or a clearly hierarchy. In life, muscles are often very irregular, but maybe for a painting, you want to "edit" life a bit to give a clear hierachy of the muscles based on size or tenseness.