Also depends on the market. Ofc the US market will favour the Mustang, EU will probably say the 109/Spitfire as the most iconic WW2 fighter.
Remember the russian market...a P-39 Airacobra perhaps, as owned by FLying Legends, to continue the ground attack theme of ED's recent years.
What WOULD be useful, and fit easily in the current world, would be a training jet and training syllabus along the lines of Captain Sim's original 'Combat Pilot'.
I'd buy it in an instant, even if it was a Russian plane (e.g. L-39 as in CP).
Not exactly a Flying Legend though...
I'd also really like a civilian Helo to complement Black Shark, again would fit easily in the current world, but with Take On Helicopter out they probably wont bother.