Basically it would be awesome if someone threw together a mod that made trees denser, and maybe changed all of the ground textures that are deserted and dry to grass, flowers, plants, and more. Just a lot of green.
Any cliffs that were unaffected could be made into grayish stone or packed dirt depending on steepness and erosion level.
We also have a "Forests of San Andreas" mod, but it would be cool if someone threw together a mod with slightly smaller trees (half or quarter of the size of redwoods) that mainly consisted of temperate varieties like poplar and sassafras, dogwood, etc. Anything you would find in temperate biomes. (much denser than normal GTA V trees) This way, we could, again, get a New England/England feel.
All in all, it would be great if someone made changes to GTA V that changed it's biome entirely from a desert to something like a lush landscape with plenty of trees, no barren landscapes, and any flat areas could become meadows or farms. Just like Forza Horizon Four or the Crew 2's east coast.