What sort of "variables" do you get from the Raspberry and how do you plan to use them to control the drone? Do you have the drone yet or do you know what type/size it will be? Are you intending that the drone will be autonomous i.e. fly on its own without any manual remote control (which is not legal in many countries)?
An Arduino can do quite lot when properly programmed but I think you need a rather more detailed specification to decide how practical your plans are.
Sooo an X and Y of the object? As in the distance and direction from the camera? Or latitude and longitude? You could certainly communicate using I2C. The Multiwii project uses an Arduino chip with a gyro and accelerometer. You could make some code changes for I2C control. However, given the complexity of this project, the Pi should probably give a direction signal instead of just the location of the object
I think the throttle should be controlled by the flight controller using a sonar sensor to maintain a steady height.
By a direction signal I mean the pitch/yaw/roll commands. Yaw should be pretty easy, just trying to center the object. Direction to move in ( given the position of the object and the quad relative to it) should be calculated into pitch and roll by the Pi. Then the flight controller only has to deal with controlling the quad while the Pi takes over all the heavyweight calculations.
I think that you could use the Spektrum Satellite protocol to control any flight control board. That is 1-way serial at 1200?? baud. That would probably be easier to implement on the Pi.
That would just be emulating the standard receiver that the flight controller would usually take signals from, and the Pi would be outputting the usual pitch/yaw/roll/throttle.
Ask on RCGroups for more specific help.