They NEED to support sound barriers and allow scram jets! Also I'm not really a developer (I have the FSX SDK but have no idea how to use it) but I was flying an XB-70 Valkrie from simviation, and it only went to about 200 knots when its max is mach 3.1, o this raises my question, does FSX have a developer speed limit, and if so they need to remove it in flight.
Also it think there should be rooster tails when flying close to the water (as an option of course as it probably takes more of your CPU/GPU).
Another nice thing would be a VTOL and JTOL support, just imagine what the developers could do with having that in the base game engine.