I can’t identify any big issues with your art, and anything I name would mostly be stylistic preferences that aren’t “errors” that need “fixing”. That said … If I had to name one thing that would be nice, maybe you could try softening your lineart sometimes, coloring it so it’s less stark black?

Like I said, this may be a purely stylistic choice, which I’d totally respect. I just feel that some slightly subtler linework might make the colors and forms blend a little better together, and give the overall art a less “sketch-like” feeling? That’s just something I personally like.