At the very least, any plane that has a fire spewing from the engine or one of the wings should get an EXTREME loss in flight performance instantly even if the pilot decides not to bail out.
Realisticly, pilot would bail out as soon as such a fire started. In game, no one cares about that virtual life, so keep flying and try to take the bad guy with you.
If fire from the engine on centerline then effect on forward vision should be even worse than oil splatter we have now. If fire from behind the cockpit, effect of rudder and or elevators would be greatly dimished in the very disturbed airflow created by the heat of the fire, similar to effect already seen for elevator or rudder loss.
If fire from either wing, consider massive loss of lift on that wing, similar or worse effect than when the wingtip is damaged.
I know its all a game, but we should be able to ignite a plane on fire and then discount it as a threat, rather than having to worry that the soon-to-be-dead pilot is still flying with no hinderence to performance of his aircraft.