The window on the passenger side is obviously for the drivers field of view. The second window behind the driver doesn't provide much vision, and most expedition vehicles have the whole of the cargo area full of gear in this area.
The solid wall area on the drivers side can be used for a multitude of expedition equipment both on the inside of the vehicle and on the outside. This may compose of internal storage up to the roof level, built in closets, water storage, shower units, cassette toilet, kitchen, etc.
The solid wall also provides a little more privacy and security.
As far as retro fitting a window, yes it could be done as an aftermarket product.
Just to clarify in my mind's eye, is this topper designed with a permanent installation in mind or removable. The reason I ask is in a removable design, how usable is this space going to be for shelving, kitchen, etc with the current placement of the rear sport cage bar as it crosses the window portal? Overall, I love the design of the topper and ultimately would like to see myself getting one, but being a fully removable design is a selling point for me. A simple shell with the flip tent would be all I need. Well that and maybe a one piece tailgate/window that could be switched out for the stock tailgate. *subscribed*