Thanks for the feedback.
Here's a "mockup" of sorts (actually in game engine) but there's a ton of filler tiles and no other art for context, so I don't know how helpful it is to look at:

(You can ignore the black rectangle, it's a placeholder for the character.)
Facet, you mentioned integrating the grass into the rocks, but I'm not sure how I'd go about doing that with my tile restrictions...
Honestly, my enthusiasm for tan rocks and green grass is dwindling fast.

I thought it would be fun to do my own take on a platform game cliche, but it turns out it just looks.... cliche.
Designing the character is something I still need to do, and I think that might give me some more specific inspiration for the environment.
I'm planning to base it on this character you may have seen me post recently:

(The ARPG concept from before eventually evolved into this sidescroller.)