You're off to a very strong start! I do have a pile of critique, but I hope it doesn't scare you! The bulk of this image is looking great.
I agree that the dithering looks nice, but depending on how the art will be displayed, it might be better to replace the dithering with a solid colour. Dithering looks good if you can't see the pixels making it up, but if the art is to be scaled up, then it's quite possible that the checkerboard pattern will become visible and look like texture rather than an additional colour. So, if this game is for PC where people will likely have it larger than 1x, dither's not a good idea. If it's for handhelds/mobile where it's likely to always be 1x, then it'll look fine, at least as long as it stays still on the screen (dithering has a tendency to flicker in motion due to how screens work).
Will there be a gradient in the sky eventually? The evenly bright sky doesn't fit the sunset look of the rest of the image.
The blue mountains could use a bit of AA where they have low-angle slopes against the yellow clouds.
The transition from blue mountains to midground feels rather sharp, I think perhaps the furthest areas of the midground (like those big brown rocks) should have some blue tint, or perhaps you could add another "layer" between the midground and the mountains to ease the transition.
The light-coloured fern in the foreground feels out of place, since the rest of the scene gets darker towards the foreground.
The dark plant in the very foreground looks good, but a bit disconnected from the scene. Consider adding some more elements in the foreground like this. Perhaps you could use them to frame the composition a bit more. Maybe you could have a more visible transition from midground to foreground (i.e. within the image and not just off-canvas) as well, so that the planes aren't so disconnected.
The bright highlights/rimlights in the foreground lack the volume of the rest of the image. Don't forget that rimlights/backlighting also follow the form and planes of the object, they don't just follow its edge. So, if your rimlight is consistantly a pixel thick, it looks unnatural, especially on flat planes like palm leaves and some of the rocks, where the light should either hit the entire plane or none of it.
The grass in the bottom left area feels rather scribbly and noisy. If you're struggling with actually rendering that sort of stuff, try working in layers of silhouettes, each a different colour.
The various 1px-thin highlights on the grass seem random and don't correspond to the lighting in the rest of the scene, especially the big horizontal one in the middle, which contributes to the scribbly look.
Probably the (literally) biggest problem: your various sharp rimlights suggest a well-defined, non-diffused light source in the middle of the image but there isn't one. At best, it's behind the clouds, which would diffuse the light and it would not create such bright, sharp rimlights. At worst, the sun looks low enough to be behind the mountains, where it can't cast its light directly on the midground/foreground scene at all. Moreover, the cloud shading suggests that the sun is either off-canvas to the right, or behind the viewer.
Lastly, there are a lot of orphan pixels and jaggies in the image. But, I think you should save pixel-polish until the end when you're satisfied with everything else. It'll be time-consuming for such a large image, but it's a perfect opportunity to put a movie or podcast on and just zone out fixing it
