Part 2: ART
So first of all, when everybody is throwing around pictures, there is a huge problem.
These pictures are like 3000x3000 resolution or something, and are ginormous... and are not representative of what they actually look like in the game.
So let's take a look at the game, from my setup with a 4K monitor. There's probably bigger ones out there... but this is pretty big, and this is what it looks like for me (I removed the UI on the sides because of what's going to happen next, so that was altered in a crop):
First off, we see some interesting things here.
Floating trees. Tail underneath the ground. Everybody is "flat" but there is "depth" to the scene, like the characters are in a 2D sidescroller but they are standing on 3D ground. The weird torch thing.
So looking at this, NOT AS INDIVIDUAL ART PIECES, but as the FINAL COMPOSITION, we can pick out a few issues that aren't obvious:
1: The lizard guy is way too dark. It looks like he's from a different game almost.
2: We have no idea what we can click on. Where do I go to move? How do I click on ground?
So let's fix this:
Update 1:
1: We brightened up the lizard to match Kay and the scene they are in, approximately
2: We darkened the background JUST SLIGHTLY, and if you notice, we now have a 'click path' for the user. Without needing to tell a user where to click, they're going to know "Oh, this is the ground, this is the clickable area where I can walk and move"
3: Notice how the bright clickable area leads into the dark doorway. The user knows "Looks like I can go in there"
4: Fucking self insert panda fell out of the floating tree apparently, with a slightly updated anatomy based on my own vauge knowledge of anatomy.
But do look how the self insert panda seems to take up more space, and isn't a stick, and blends in a bit better because everything else is wide and big. Also note how I attempted to make her feet look like she's standing on 3D terrain instead of a flat side scroller.
Let's keep going though, we can ride this train a bit more.
Update 2:
1: Since in reality of the game, everything is so small, it's actually really hard to pick out "details".
2: I added some extra color to make Kay stand out more and give her more depth and not look washed out.
3: I added black lines on some of the details to make things look like you can see them and prevent everything from looking "washed out". I outline some of the equipment so it stands out and it looks like something.
And finally:
Update 3:
1: All I did here was basically make some colors stand out more, and applied a Posterization layer to the characters.
The color was to make their important bits stand out, and things like the scar got an outline because that's what "defines this character" as being different (I think).
The posterization was to basically show that, in the game, you can MASSIVELY reduce the level of shading you are doing currently to come out with effectively the same looking thing. This would help reduce your work flow, and likely make all of the spine based animation work better since you don't need to worry about so many gradients not mixing together well during animation.
I think though, most importantly, as a final aspect: This LOOKS LIKE A VIDEO GAME that you can just glance at, know what there characters are, know where they are because they are clearly defined, know what each piece of a character is, and know what you can click on and how you would navigate the scene.
Ultimately, you can't have designers "doing their own thing".
The environmental artist needs to work with the level planner, and they need to mark the clickable parts somehow so players know what to click on.
The characters need to look good for the scenes they are in, not in a Photoshop white and grey background.
The character designers need to work with the environmental artists so they can all match tones and hues, so we don't have super dark characters or super bright characters, everybody needs to agree to a color and lighting standard.
The characters don't have enough color variation, so while those subtlties of color look great in, again, a photoshop background on their own, inside the game you can't even tell "what is hand???" when looking at a character as it is mostly gone.
All those subtle effects are effectively lost inside a game, and you need to really give some things "impact" so it will stand out and be visible. This means a lot of contrasting colors.
Use these contrasting colors to also train your players on how your game functions. Nobody should wonder "is this something interactive?" it should just be "oh, this is something I can interact with, cool".
Nobody should ever be stuck or lost at "simple navigation". If people don't know how to just move their character around, you're going to increase frustration.
In the software development world, we manage "frustration level". If something has too many "frustrations" to it, that thing is going to be abandoned and never used. If too many critical things are hidden in frustrating areas, you're going to see huge abandonment. You will get dunked and not many people will play or enjoy your game/product.
I know you want this to be "a real video game" of sorts, so plan accordingly!
I hope this was helpful.