Already looks pretty good!
I personally would add some high res textures to your vegetation meshes -> atm they look very blurry (could be caused by the mip maps) + do you use the foliage shading mode in your material? + on the 2nd last picture you can see a normal error (flip them in your 3d program) 
I personally would just work on one of them, because then you can place more meshes (and high poly ones) without a huge performance loss -> but it always depends on what you want to create