Your art is great! However I would suggest the lines be thinner, try a size 4 pixel brush. If you don't already, i recommend working in layers and if possible, use lock alpha when you shade. it keeps it clean. Also, add depth to your drawing and shade it. When shading, try not to use striaght up black too much. try using a deeper, darker tone of the base color from the original colors you used to color it in. for example, if you initially colored in a part of your drawing with a yellow, try a light orange or brown color to shade it in with. When doing an outline, keep your lines in a continuation. If possible, try to use the stabilizer for a cleaner shape. Don't be afraid to use things like the circle tool or line tool every now and then. Also, this is one of the most important things i can say to you. Wherever there's a shadow, there is a midtone and a highlight. Don't add a shadow but no highlight. Depending on the lighting you're going for, you may want to stay away from super bright whites as highlights. for softer lighting, go for a lighter version of the base color. (sometimes it is okay to use black and white but dont overdo it) I will include a diagram so you can see what I mean better.
You have good creativity and good concepts. your art looks nice ^^ keep up the great work :D