Here is a list to get you started on the minimum of inputs the developer would need. Keep in mind that this is only for the calculation of vehicle dynamics.
WCS = World Coorindate System
BCS = Body Coordinate System
What is needed from the engine
Atmosphere Data (at object's current location)
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Altitude above sea level
Temperature
Speed of Sound
Density
Pressure
Wind in WCS
Any other weather parameters (relative humidity, etc.)
World Data (all in WCS)
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World position
Altitude above ground for any location (BCS) on the body
Ground surface information (if available)
Orientation (quaternion/euler)
Linear acceleration
Linear velocity
Angular acceleration
Angular velocity
Body Data (all in BCS) (Saves the developer the trouble of converting all world data into body data, but I suppose this would require knowledge of the developer's choice of coordinate system and orientation)
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Linear acceleration
Linear velocity
Angular acceleration
Angular velocity
Wind in BCS (Could be included in linear velocity, but may want to keep separate)
What we send back to engine
Mass Information (at each time step)
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Mass of the object
Moment of Inertia in all axes
Center of Gravity in BCS
Forces/Moments (function for each)
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Local Force (X,Y,Z) in BCS at location (X,Y,Z) in BCS
Global Force (X,Y,Z) in WCS at location (X,Y,Z) in WCS
Local Moment (X,Y,Z) in BCS
Global Moment (X,Y,Z) in BCS
and/or a bool function to continuously pass forces (instead of the developer summing them) during the time step until returning false
There are other calculations that you could do to make the developer's life easier, but they are specific to aircraft and I think you want to keep the interface as generic as possible.
If someone implements a flight sim on outerra, I suppose they could take these essential inputs and calculate other things that someone might need.