I don't have the place so likely I never will buy one (also because I rent a flat and I have the faint feeling that my landlord would not be too enthousiastic about me screwing the thing to the floor).

I think adjustability and modularity is the two most important thing with ranking in just this order. Adjustability for me is the key to comfortably flying. Beware that your devices should fit to a wide range of people with different sizes, ranging from kid size to giant size.

Adjustable functions would be for me: distance of pedals to the stick, distance between the two pedals, height of the stick, height of the pedals.

I would like to have the brakes on the pedals (independantly moving). For the Spit however you would need the brakes on the column so this might be incompatible with modularity.

Force Feedback is essential but one should be able to turn it off.

However, if you really want to stay authentic it may be limiting in the buttons that can be integrated into the throttle lever or the stick, so perhaps you could offer as an addin some panels with programmable buttons and axes that can be easily attached to the frame.