Hiya
Cool piece! You seem to heave put in a lot of effort already!

First thing, I wasn't sure what I was seeing. I thought it was some kind of swamp with water, and ghosts shooting through the water... But after looking at it for much longer, I'm guessing we're seeing the world upside down?
If the latter: Why are the bottoms of the trees dark? Since they point to the sky, they should be catching light right? While just around the edge, there should be no more light coming in, while on the front tree (the one the guy is standing on) the edge is even highlighted! Where is light coming from?

Also it is hard to read depth in this. The sky is so different further away,  it would almost seem as if it climbs uphill or so.

I think the animation can be much improved too. I like the effect on the cape with the various wind speeds, the increase and then drop. You coul synchronize this with the swinging character, who could speed to the right fast and then drop slowly back to the left, never really reaching the left side of the pendulum.
The cape it self offers a really nice opportunity to have nice waves going through the fabric. Now the curves are really random, try to have them all move from the left to the right, like a ripple going through water. You would probably have to redraw that. You can then have the edge be influenced by where the waves push you.

Good luck with this!

Thanks for the reference pictures. It's still not clear to me where the light should come from then, but in this one it makes sense that the bottom of the trees is well lit.
http://www.anime-evo.net/wp-content/uploads/2017/08/Made_In_Abyss_05_05.jpg
I think you should do that too, and remove the thick black outlines on the bottom.

As for the cape, draw just the (bottom) outline and animate that. Draw one frame, count 12 steps between each wave and then draw all those 12 in between steps. Don't space those 12 steps equally, put 5 really wide and 7 really narrow to get the strong wind various speed effects. After doing the outline it should be relatively easy to do the shading in the cape again.

Cool, albeit a little crazy artwork! My main nitpick is that  these trees look more like mushrooms, i.e. their cups are too perfect.

Edit: ooops! just noticed your reference. Guess they are indeed mushrooms, with tree-like trunks. Still these anime stills show less perfect cups, and they have no leaves

Also, now, when I watched a few concept design videos, I could also notice a few problems with general composition idea. I.e. the picture is too messy, disjointed and not to the point: the general idea seems missing. Are these creatures friendly or what? And the character is standing without much purpose. For example, the mood would drastically change, if there was a climbing creature directly behind the character and the character itself was more wary.  It would be a nice idea to blur out background elements to attach viewer attention to important detail. I.e. use less frequency details on them or some fog.

I've added a few colors to more obviously put the other trees further into the sort of foggy background and roughly sketched in some corrections for the back trees for a start.

I don't have a mouse with me, I would have wanted to make some corrections, but now I'll just try and describe a bit instead:

It's a good start, but there are a couple of weird things and jerky moments.

The end corner on the right should respond to the wave, not lead a life of it's own. It follows through on the wave as it's incoming, but because it has less weight it might make the move more exagerated. Now it often curls in the opposite direction.

Frame 8 suddenly is in a completely different position than frame 7.

The top of the cloak should move much less jerky. One time up and one time down stretched out over the 12 frames should probably work.

Ideally we'd be able to see this last blow before the wind calms down for a split second, we'd see that ripple through the cloak. There are like three beats going through the cloak now, three waves of wind come through every cycle. If it were one wave the cloak wouldn't ripple like that. But in the cloak there are 4 waves present at all time? I'm not sure if I'm completely right. BUt it does seem strange how these curves remain during the bit of rest. Of course some curves can exist in the cloth itself, like in a curtain, but it does seem kinda stretched near flat in the wind..

The cloak on the left top, that's coming over the shoulder around the arm, comes down in a very sudden way, maybe that should ripple down from the top a bit?

All complex to explain... There are quite a few flag in the wind references/animation tutorials out there, maybe have a quick look through those?

I also stumbled upon this video, which seems useful. And this reference!

To be honest, there was a slight language barrier with Hapiel's first explanation- I thought he meant the curves by waves and was thusly very confused, but still tried to apply that misunderstanding to the correction. (I feel in german I'd basically describe the curves as waves and vice versa).
I hadn't changed anything on my old starting file except the bottom outline yet but wanted to check in again because I knew something was off.

I'm familiar with the wave principle in, well, principle. Thanks for the time of you both in searching out these videos, but sadly, video references/tutorials are like brick walls to me. A friend owns a few books on animation that I'll try to borrow though, and Hapiels explanations help tremendously now that the wording is clear to me. Thanks again!

I reduced it to the bottom outline by it's own for a start:





There's some jank in all of them that I'm well aware of, but I'm trying to get the different wind speeds down again in principle first. I'm obviously really having trouble closing the loop, but I'll keep working on it and update this post if I feel I've got something better to show before anyone replies.





As for 3D animating it first, I'll have to decline there on the basis that I can 3D model alright, which I learned at university over the course of a semester, but am not familiar with rigging, animation and scripts in any of the 3D-software I'm familiar with (ZBrush and Cinema4D). This means that I would have to aquire one, or arguably multiple, new skillsets in order to create a reference to practice improving a correlated skill.
Of course I want to improve the animation, but my main intention was channeling my respect and admiration for the story and character this is related to, which is why I don't see the animation as integral enough to warrant an investion of time that immense.

I see the value of 3D as reference and as a medium, but still disagree on it's indispensibility, snv. I'm not here to argue, though. Thanks for your input.


Edit:
I think I'm getting closer.

Yeah. That is nicer and less jerky.

You have enough dull purples in your initial 28 colours to achieve some nice fog effects, I think. I don't think you need to add more. You would lose some detail behind the fog by reusing colours, but I don't think that's a bad thing at all - obscuring detail is exactly what fog does.

Very good progress here! Just wanted to say that I think the clouds looked better when they weren't as bright. The white part looks kinda pillowy and distracting from the central figure. I also quite liked the pink dots you had in your background trees before.