So, here's the deal.
I want a push-pull twin with tandem seating and significant luggage space, including a retractable gear. Long range (1500NM), ultrahigh cruise (225 kts or so) is the design goal plus reasonable take-off performance (1500 ft roll, 2500 ft over obstacle sounds good). Engines should be the Rotax, Jabiru or (most likely) the BMW R1200S block, all around 80 kgs (175 lbs) and 100HP, including everything except prop. Mid-wing, to make aerodynamics both optimal and easier. And not to forget, removable wings with a maximum (naked) span of around 6' 5", so trailerable. Why a twin? More safety for crossing large bodies of water (like the Atlantic and Mediterranean), cheaper cost (a bolted BMW is about a quarter of the price of a Rotax, let alone a 200HP continental or so) and also, it's different which is also good ;-)
What about the best configuration. 3 are common and realistic:
The canard with a front engine added. The Defiant is the best example.
I don't particulary like the idea of a canard. CG range is narrow since the wing has to shift forward (100 kgs extra in the front) and the canard has to be really big anyway because of the limited distance between main and canard. A big canard means very low visibility. Flaps can't be added so either a big wing or very long T/O length.
Second one is the twin boom as used on the Cessna 336/7 and the Adam A500.
Spar location is a bit of an issue (midwing), so the passenger probably has to sit just in front of the spar. Also the tailboom adds quite a lot of drag. The (RG) landing gear can either be stowed in the fuselage or in the booms. Stowing the LG in the booms is an option, though they have to be pretty big (and draggy) to stow even the smallest tires, let alone some 6" grass-capable ones. One thing I particularly don't like about this is the added complexity, double controlls running tru the wing, landing gear assembly going tru the wing. Also propellor diameter is a major issue since the booms can't be too wide apart (trailerable) which also includes the problem of an exploding prop cutting of the tail and fatigue because the interference between prop and boom. Wet wing tanks is almost impossible due to all stuff running via the center wing.
Another issue is the massive turbulent airflow over the elevator when the aft engine quits, exactly then when you need attitude control the most..
Third is the push-pull with controls at the tail like on the DO335.
I personally like this one most. Really simple construction (simple is good and safe) with a complete wet center wing and simple plain flaps. Landing gear needs to be rather long, but stowing it in the fuselage (Extra-400 or F-16 style) seems a good option, also making a great crash absorber in case it does fail.. Turbulent airflow entering the rear prop is an issue since deflecting an elevator/rudder provides a lot of turbulence. An alternative is a T-tail. I probably can do without a longer driveshaft since especially the BMW is rather slim and light and it can thus be positioned and the rearmost point. Avoiding the driveshaft is a major advantage (vibrations!) A major issue is the point of gravity, there has to be some heavy stuff behind the spar, like a landing gear, an emergency chute and tail surfaces. Even though the tail will be rather long, comparable to the "Pfeil" Because of prop clearance the landing gear has to be rather long. We needed weight in the back anyway
This configuration also leaves a lot of room behind the main spar to stow luggage (light and volumious).
So, which points did I miss and why is my preference for the Dornier arrangement wrong? Why would you prefer another push-pull configuration?