I would expect the more i move the mouse , the more it moves on Z or Y, the same proportion it moves good on X axis.
It's a downside as many times, i am not using top down or orbit, but any view and i ,eed to be able to move proportionnaly on X/Y with mouse like 3D scultping or modeler software are doing.
It's really disturbing and annoying to not have that uniform movement in a 3D engine
At least make some option to let people choose traditionnal standard uniform X/Y movement, or the very special Skyline one
(Like quick front , side views and orthogonal perspective are really missing when you big level design)