Every Ork Player comes up with his/her own creation(s) for Ork Vehicles. Many of them are great! And I believe that G.W. should loosen their tight grip allowing Ork Players to use their artistic license.
But what if this becomes abuse?
Let's say an Ork Player creates a HUGE Battlewagon and puts a MekBoy with Kustom Force Field in it? The vehicle is SO BIG that the Kustom Force Field's 6" Range can cover his/her entire army!?
And then, another Ork Vehicle is SO SMALL that line-of-sight is nearly impossible to draw to it.