Hey man, first off, what you've got is already really nice! Great vibe and I LOVE The design of your little blue dudes. I made a paintover to explain what I would work on if I was finishing this piece. I'll try to make a list of the major things I did.
- Blurred all the flowers. That kind of high frequency detail will create unintentional focal points.
- Blurred all the edges in the background. The sharp edges were pulling the background shapes into the midground.
- Added atmospheric perspective to the middle hills, and ever MORE to the back hills and behind the bridge. This is important to separate foreground, middle ground, and background. The one behind the bridge is especially important because the BG there was originally too close to the bridge in value, making the bridge blend into the BG. The atmo perspective is lighter so now the bridge pops out a bit more.
- Darkened the foreground slightly (bottom of the picture). Hopefully deemphasizing it so that the viewer will look more at the central part of the picture. Also darker values help things "come forward" a bit more.
- Picked a couple focal points near the "rule of thirds" intersections to pop out a bit because the pic felt slightly flat. I went with the bridge and the waterfall/grass arch. I added some detail, and upped the contrast in these areas. Also added a yellow/orange overlay on the bridge to get some hue variation so its not all one hue.
- Added water detail to the waterfall. Just a few bright reflections and some extra lighting under the arch. Added some lighter values along the edges of the water where it meets the cliffs to make the water look more "watery". Also exaggerated the cuts in the cliff for more visual interest.
- Added a yellow/orange overlay on the cliffs to generate some different hues so they're not all one hue. Also moved them a bit more towards red/brown as they seemed a little too purple.
- Added lighter, sharper detailed grass blades to the top of the arch to pop it out from the ground behind it. It was blending in before. Also darkened the grass behind it for even more contrast.
- Did a similar thing on the center bottom grass that overlaps the water, same reasons.
- Darkened some of the path to provide some visual interest, draw the eye to the blue guys a bit, and just because it felt too bright. You could get away without this, particularly if you need the path to really stand out for gameplay reasons.
- Darkened the ground behind the left blue guy to separate him from the BG more.
- Made the water more "ocean" blue to match the "everything is awesome!" vibe. It was a bit dark and desaturated and kind of purple.
- The cloud in the V cut behind the center blue guy was creating a really high contrast area and it prevented the cloud/hills from receding into the background. I extended the hill to block the cloud and moved it over to the left of the "spire" rock.
- Threw a yellow/orange overlay radial gradient in the middle just for kicks to get some color variation and highlight the center even more. Pretty over the top but it is a cartoon right :)
- Slightly upped the saturation because cartoons :)
- Color balanced the sky/cloud top third of the image a bit bluer to push clouds/distant hills a bit more into the BG.
- I didn't do this at all, but I would tend to agree with Admbrns that you'd want a nice smooth gradienty look without visible brushstrokes. Also in general some areas appear a bit blobby now due to soft brushes; that's probably okay in some places but in areas near focal points you'll want to make sure that you have sharper edges, especially for stuff like grass overlapping dirt.
Dunno how much of an improvement it is, but hopefully you can get some use out of this! Keep rockin man!