I think you've got a good thing started here! I like the bright colors and simple shapes. It kinda reminds me of Wind Waker, which I love. That said, my suggestion for improvement is to look at the art from Wind Waker / Phantom Hourglass and see what it does. For example...
Adding some simple shading would help your characters pop. Using dark colored lines instead of black, especially on the interior of the character, would help lighten them up. I would also suggest practicing loose gesture drawings, both from life and from any other art you like. Try to look at how the characters are posed and how you can sort of imagine a curved line going through most of their body. That's what helps their pose look dynamic and appealing.
Keep it up! It's looking good!