Cover Illustration

Hey all! I need some advice. I’ve been striving to teach myself better illustration and want to take this design up a notch. I was thinking of adding in some texture (but unsure on how to go about this)? I will be adding a little more line work, but no outlines per brand guidelines. Please help me be a better illustrator!

*side note: the blank space above the bus is being held fife verification of use of a specific element by client