art - How can someone make 3D tiles not looking like perfect cubes?

Here are the graphics an artist did for me:

While the texture is nice when seen as image file, I think the tile is somewhat flat.

The problem is:

If he bevel the cube, then there will be tiling problems (not to mention seams between cubes). How can he make tile looks less flat?

If he make the 3D model to act like 2D isometric Tiles (which have very irregular outline) there will be so much polygons that will be a nightmare for him to model dozen of Tiles.

Of course keep in mind that for occluding reasons (each cube project 6 planes that are used to occlude hidden geometry) the overall shape should be actually bigger than a cube volume, but not smaller (anyway I can loose the height requirement, cubes can be made of any height).