The coloring job on this one seems great, and the mane, tail, and cloak are well done to suggest that Trixie is gonna cause quite a mess.
Sometimes, part of making great art is not just being able to use space, but being able to make use of unused space (i.e. negative space). We could move Trixie to the right so her eyes are 1/3rds of the width of this piece from the left edge. We can easily think of the edges of a piece to be a little wall, and cropping too closely to a character can make a piece feel... cramped. Moving Trixie with compositional techniques in mind like that should probably make this piece feel more dynamic.
Anatomy-wise, we ought to be more careful about leg proportions. They are differently shaped. The back legs are also proportioned to look like human legs. What would the legs look like if you drew what's cropped out? Whatever it looks, it would look really odd and will probably look longer than the front legs. I also think Trixie could benefit from having white teeth considering the sort of coloring style you went with. If you didn't shade this, leaving them uncolored might work better IMO.
Overall, it kinda feels like one of those pieces I can't really have any strong opinions as this isn't particularly bad, save for the slightly cramped cropping to our left and some weird leg anatomy. While I can't be completely sure how moving Trixie to the right would change my impression, but if there's still something missing, there's the possibility of including a second character to create a little story with just one image.
Trixie: no one is more powerful than the Great and Powerful Trixie!!! Me: there is one more powerful thatn you! Trixie: who can be more powerful than the greta and powerfu- Me: StickFigureQueen!! she draw you!! so ha! ther you go! Trixie: ...ok...