I want fire to melt the wires to your trigger button. That is, you can no longer fire fixed guns when on fire. Bombers can drop bombs if only to pickle them -- but I say this only because I'd like fire to do more than it does.
I want a fire to destroy things around it. If it's in a wing tank, I want it to disable the guns in that wing, destroy the gear in that wing, destroy the flaps, destroy the oil cooler (if spit/109 type of wing oil cooler).
I want it to do this gradually. THEN finally the wing pops off. So, instead of this "long wait then BOOM" I want "you can't fight and keep losing stuff every 15 seconds" -- this gives you a chance to go and set 'er down or RTB and get down in time if possible. However you still lose that part if you wait too long.
Here's the fun part: If it's a fuselage fire, I want it to be able to set bomber bombs on fire! It could be one of the parts that get "damaged" -- and the bomber has to pickle all the bombs before they blow.
Center parts that could get burn up include other fuel tanks, nose gear, radiators/coolers (depending on plane type and location of fire), and on single engined fighters: the PILOT will get wounded (just as shot) if the fire continues too long.
The overall time to death would extend, but the fight would be over instantly and it would be a race to survival.
THAT is what I want to see with fires!
EDIT: As the parts "go" you could just hear the "loud bang" noise in AH already and the part goes red on the damage list -- it's a very simple thing to include IMO, don't need to add any graphics!