Trails of any kind are an excellent visual cue to judge an enemy's E state.
This is implemented very nicely in the current version as well. If I happen to fight a fighter trailing radiater, engine oil, or fuel leak vapor you can tell how fast he is by the length of the smoke trail.
Its an excellent indication for me to know if I have enough energy advantage to take the fight into the vertical, with the hope of having him either attempt to follow me and stall out below me as I reverse down on him, or he will just stay below as retain my advantage and set up for a B&Z pass.
Thick black smoke behind engine fires would be very cool, and a blast of thick white smoke for a few seconds after an extinguisher has been discharged would be even cooler !

Only a few WW2 a/c had engine fire extinguishers (mainly bombers) and would be an extremely cool item if implemented.