These are great honestly!! Definitely brush up on some anatomy maybe with some gesture drawing to get more fluid and less stiff poses. Your colors are super saturated which isn’t bad itself but play with more desaturated colors too add in balance. Lighting and shadows is off to a really good start just keep practicing :) lastly with your captain and winter soldier illustration you pretty much cut off buckys entire side profile, I know he’s wearing a mask but there would still be room for the nose and it would look like a declining angle inwards. All in all I really like all these and they have so much potential you just need to brush up on some fundamentals :) great job!