It is already possible to set the x- and y-axis to use the same length of the axis for the same difference numbers. (set size ratio -1)
It is also possible to make the z-axis the same length as the other axis. (set view ,,1,1)
It would be great that one could specify, that the same length in the x- and z-axis represent the same difference in numbers, (for example by allowing negative values for set view).
Both 4.4 and 4.5 accept the new command
set view equal xyz
I think this does what you want. It has a problem, though, in that it doesn't work on all terminal types.