Did you ever really use a DatumPlane to map a sketch? They very well have an origin!
It's not a question of control, it's a question of visualisation to know how the plane is oriented before it is used.also, map mode pitch and roll of both the datum plane and the sketch (or other objects attached to the plane) do already give the user the needed tools to have control over this.
Here are images of three different(!!) DatumPlanes as to be seen while editing. What would you do to tell the difference? I attach the file too, so you can verify the images.
I cannot see what will be confusing or wrong to show where (0,0) is on the plane or in which direction x, y, z go. It seems to me that you want something you know from some other system, which cannot be mapped directly to the situation in FreeCAD.