We already have the Q and E buttons mapped to flying the camera up and down. Right now, I have no way to move the camera forward and back whilst looking down, without simultaneously flying down.
The click+drag method of flying forward without changing height has its own drawbacks.
In my view, the ideal solution would be to give a toggle in editor preferences to treat W and S the same as A and D (ie. all four keys move the camera horizontally only).