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.