Seems like you're finally trying to make more complex works! Glad to see that! :D
The lineart is a little too thick in some parts and too thin in some parts, this is not a bad thing, kinda reminds me of Clamp's artworks, but some lines are just to much thin so they are clashing a bit with thick lines.
And I want to ask why you didn't use lineart of birds? their wings are looking like a part of cloud to me, also for lining the clouds you should have used darker blue/grayish color, and blackish for making line of birds, that way the birds and clouds will look separate from each other.
Also blur the clouds that are in distance, to make the wall more deeper.
And also a thing, why sky is just a solid tone of blue? sky has many hues of blue, using gradients, some brushes and then smudge them will help to make the sky more lively. (as in original scan the earth is visible on left side, that means Miku is on pretty good height that's why sky has darker tone of blue because the space is going to start from further up, but here in your wall you didn't made the earth, so it should be supposed as normal sky xD)
You're going better, just focus on lighting dynamics and also use another layer of shading as gradients, they will make
your vectors more pretty!
Keep it up!