I imagine a fixed wing airplane design that's a lot more symmetric than what I see today. This symmetric design has wings in the middle of the fuselage (neither high wing nor low wing), and the empennage is a symmetric cross. Two vertical fins, one on top and one on bottom, and two horizontal stabilizers. The tail fuselage would taper symmetrically too, instead of the underslope with flat top we usually see today. This would make the fuselage closer to a Sears–Haack body, which I believe is ideal.
In other words, it would look a lot like the XB-42 Mixmaster:
Ignore the weird engine. AFAIK, this design could use an engine anywhere. BTW, this is the only aircraft I know with the middle-wing and symmetric cross tail design—whether experimental or production aircraft.)
The reason I imagine this is so the control surfaces are much more aligned with the center of mass. So the rudders, elevators, and ailerons will not produce any adverse torques when used. I think this will be more efficient, especially when using trims, and also is a simpler design as far as I can tell.
To get around the problem of tail strikes, I imagine a single tail wheel attached underneath it. For small aircraft it might not need to retract. For larger aircraft, I don't know how or where it would retract to, but maybe instead some kind of aerodynamic shield can enclose/open around it, like a prolate or oblate spheroid shape.
(An alternate symmetric design would be an X-shaped empennage—same shape, just rotated 45 degrees. I think for a given yaw, the combination of control surfaces would produce more drag though, but it's a way to increase ground clearance.)
(A more exotic alternate might be to forget about takeoff rotation entirely, and just increase takeoff speed and the length required to literally lift off without pitching up the nose. This might not be feasible for large commercial aircraft though.)
(But maybe best is to add canards with their own elevators, or all-moving canards, which will push up the nose and pivot at the wheel.)
To me this symmetric design seems simpler and versatile. By versatile I mean it looks like it could be used for small and large aircraft, whether prop-engine or jet engine, military or civilian, and not really mattering where the engines are located either.
Is there anything wrong with this design? Why is it not used more often? In fact I do not know of a single production aircraft with this design.
If anyone knows of any aircraft in this configuration, whether experimental or production, please provide a link so I can read more about it and maybe discover some reasons for its success or failure.