The side panel is the finest example of where manufacturers should offer options

Plain, windowed, small fan, large fan, row of fans all come to mind.

What would also be interesting to see is somebody building a side box option, eg a side panel say 50+mm thick, for watercooling guys to place rads and fans in.

For air rigs I would say that side fan position should enhance the front fans by adding flow where they are weakest. this is often over the memory area of the board