I like what you have so far. I think there are two main problems with the sprite; the colours and the pose. Bowser doesn't suit an upright pose like this, you can bring the legs further apart, and have him leaning slightly lower and more forward. Give the arms a more aggressive position too. Something more like
this.
With the colours I'd say you need a little more contrast, possibly a little less saturation, and some hue shifting (search this term if you don't know what it means.)