Looks like you have reached the current limit of improvement in this style. It's cool, cute, colorful and distinguishable . If you want to move on, try something different sometimes - academic drawing is probably the best choice. I think you may also benefit from landscapes or architecture, or maybe changing the medium. Observe nature, think and experiment! It may be hard to step out of your "comfort zone", but it's really worth it and fun. Your current style will benefit from this too (anatomy, backgrounds, color and stuff).
You already are pretty good at what you do, the problem is that your atanomy isn't very realistic. I know you may think this is crazy but even considering the fact you have a very cartoony style you still should think about studying actual bodies. Even chibis need to have human anatomy, then once you understand it you can easily tweak it to make your own style.
So before you go too far you should take a step back to study real human anatomy. Hope I helped!
I think with your current style you can't improve anymore, because with the shading and everything it looks almost like you've mastered it. If you wish to improve your over all drawing skills however, you might try practicing other styles, which could help out with your current style.