That image is nice in the way that it depicts that secret desire to fly freely in the air that most of us have, even if the image in question accomplishes that with psychic powers, but I'd say it still counts. Her body has some pretty thick black strokes that add a nice variety to the lineart, but they're completely absent in the hair, so adding a thick outline to that part as well would add some nice consistency I feel.
As for what your art is lacking, only the artist themselves can generally find an answer to that, but taking a break to focus on something else or being inspired by the art of others can definitely give you a better idea of what you want to accomplish with your art. Or if anything, knowing what you'd like to be able to draw in the future.
Best of luck in your future endeavours. We have an art system in place here at Smogon where one can get approved to illustrate some of our demanding stuff, either by applying for it or just being approved directly. Given the quality of your art, you now belong to the latter group.
We have a page with more info about this, but the gist is that if you wondered about helping out with
Smogon's art demand, you're now in a position to do so, but with no sense of obligation to it.
But then again, if your fanfiction is as good as your art, then you'll probably have plenty of eggs in various baskets to be occupied with. Thanks for giving our corner of the internet a visit.
I tend to upload images to imgur.com where you have the option to edit the size/dimensions of the image once uploaded. Another solution would be to simply save the images in a smaller size on your computer, or as a smaller version if you want the picture in its original dimensions readily available.
Can definitely echo these sentiments as well. Even if the lineart remains dark but just have a slight hue that fits with the solid colors and shading, it'll do wonders to make the drawings more immersive, much like shading with cold and warm colors. One tip I can give about backgrounds is that you don't always need to fill the entire canvas with 'em, just enough to hint where the characters are but then leave the rest of the image white/transparent so that you won't feel the need to saturate the whole canvas with details.