One problem that i saw as soon as i turned on the engine is that the aircraft operates like a CTOL (Conventional Take Off and Landing) aircraft. All aircraft in the Harrier family (including the AV-8B ll) are V/STOL (Vertical Take Off and Landing/Short Take Off and Landing) aircraft.
Which means that the Harrier is supposed to take off straight up and then go forward (Like a Helicopter), or taxi along a short runway and then climb, not taxi along a LONG runway and then climb.
So in order to meet this requirement, You would have to develop a Vertical Take-Off system. If you want to be super realistic the method the real Harrier AV-8B ll uses involves a single turbofan with two intakes and four nozzles.
