You're off to a promising start! There was one star under a ledge that I was having a hard time getting to and it was becoming cumbersome to keep circling around to attempt for it... So not sure how much more is past the first door, although that fireball in the thumbnail looks cool.
This appears to be made with GameMaker and I'm wondering if it's running as fast as it can in my browser or if it could run smoother. You could try setting the room_speed to 60 and see if that makes a difference in the browser.
Sound effects and music would of course be great, the first three sounds I would plug in would be the jump, the stomp of an enemy and the collection of a star. When you land on an enemy, it would be great to see their face turn sad and give them a quick squashing animation.