To my mind, the core of an excellent flight sim is flight models, damage models and human models (i.e., AI behavior).
Pretty terrain and photorealistic graphics are secondary to getting the planes to fly right, fight right and take damage right. The third aspect is creating realistic aerial "terrain" in the form of weather, visibility effects, and so forth.
Otherwise, you're just creating an airborne FPS like War Thunder.
While IL2 has its faults in all four areas, right now it's the best we've got for WW2.
What would get me excited about a new flight sim would be improvements in the areas I've mentioned as compared to IL2.
Flight Models: Dynamic Center of Gravity, Realistic engine, fuel and oxygen management, Realistic radio management, fogging of windows at altitude, Possibility of systems failure, Truly amphibious planes, Realistic water landing and ditching, Skidding on mud and ice, bogging or ground looping due to mud or deep snow.
Damage Models: Custom ammunition belting, Better consistency of damage models (e.g., some planes are much better at surviving damage than others, even though they're of about the same weight and construction, the same engine in different planes survives damage very differently), Better fire starting and fire damage modeling (self-sealing tanks are seriously nerfed), Realistic damage texture modeling (i.e., damage textures based on type of bullet and hit location), Better critical hit modeling (to crew, guns, fuel and oil systems, oxygen systems, control surfaces and control lines), Realistic injury to crew (i.e., hits by large caliber bullets should be lethal, as should nearby hits by canon shells), Realistic smoke based on exact damage, Realistic coolant/oil/fuel leaks leak, Fuel shut off and diversion, Manual fuel pumping, Manual fuel dumping (for the planes that had it), Realistic oil/coolant/blood splattering.
Human models: Human reactions to hypoxia, frostbite, fatigue & combat stress, Ability for AI planes to get lost, Ability for AI to "recognize" landmarks on the ground, Ability for AI to accidentally attack friendly targets based on familiarity and type of target, Ability to command bomber crews fully, Ability to perform first aid on wounded crew, More intelligent info from crew and squadron mates (like calling out locations of enemy fighters, ground targets, parachutes, etc.), More intelligent AI crisis management behavior (like deciding when and where to bail out, or whether to bail out, ditch, or crash land), Realistic flight, squadron and group tactics based on year and air force, Realistic ground control/tower interaction.
Environment: Having a map for the entire world, or at least a big portion of it (like was done in FSX), AI for ground units, ships and AAA, Submarines that submerge, travel underwater and surface, player controls AAA guns, Ability to place real trenches, craters, etc. which make holes in the map, Dynamic weather with the ability customize cloud types, altitudes and percentage, icing in cold weather, Dust storms, Different levels of dust kicked up depending on terrain type, Partially destroying buildings, more realistic building damage models, Larger fires, Fires that spread on the ground, Wave/water action and appearance based on wind and weather, simulation of boggy/muddy ground, simulation of deep snow, simulation of icy/slippery conditions, contrails for prop tips in humid conditions, realistic radio, radar and navigation systems.
Perfection would be the the Air Traffic Control, Dynamic Weather and open-source of FSX, damage models of something like Rise of Flight or Cliffs of Dover, Flight models of CloD, RoF or BoS (or maybe X-Plane) and the content and focus of IL2.
Finally, I don't think it's that hard to bolt a new graphics engine onto IL2. It's been done before with IL2: Wings of Prey. It would certainly make the game look better, but as I said, it's not how the game looks, it's how it plays that matters.