Your desert sands look pretty good. A few items of interest to break it down such as the grass as you said and the figure will set it off for sure.
Square or rectangular bases are the most popular with model makers. They are simple to make and can easily be framed with inexpensive picture frames or if you are using styrene sheets for height, the square and true sides are easily covered by thin balsa wood or plastic card and then painted. If I were making a large diorama I would certainly do a square or rectangular base. However, for small vignette's such as you are making, I would do anything other than square. I think in this case such a shape designed to fit the model and figure would look more pleasing to the eye. I stress Pantherman, this is just my personal taste as others will no doubt disagree. Also, have you considered adding a few small items of interest such as a road sign or discarded equipment or even a few rocks to break up the empty spaces? Again, must my opinion.
Phil