Так всего лишь надо повернуть вектора джойстика с учетом камеры.
//Примерно так
Vector2 JoystickPosition = _Joystick.GetComponent<EasyJoystick>().JoystickTouch;
if (JoystickPosition.magnitude != 0)
{
Destination.x = JoystickPosition.x;
Destination.z = JoystickPosition.y;
Destination.y = 0;
var cameraForward = Camra.main.transfrom.forward;
cameraForward.y=0;
cameraForward.Normalize();
var rotation = Quaternion.LookRotation(cameraForward);
Destination = rotation * Destination;
_Sphere.rigidbody.AddForce(Destination * Time.deltaTime * ForceMultiplier);
Debug.Log(Rotation);
}