Even though I have no intention of buying another new system in order to play FS10 at the very minimum settings (max settings will probably require tech that won't be available for another 3 years! :)), I have a few ideas.
I'd like more ATC - like Radar Contact V4. I'd like ATC vectoring a little better, sidestep dual approaches into KSFO, ATC requests for speed changes for better spacing in tower space, etc.
I'd also like the ability for airports to close in extreme weather. Maybe you takeoff from KLAX and by the time you get to the east coast of the US, the weather is so bad that the airport has closed, like many winter storms have done at KBOS, and you have to divert.
Also, more airline specific details, like in the above mentioned diversion, have the airlines determine where you go to move their pax.
And one final request - JETWAYS! :) There's nothing like getting to your arrival airport and not knowing where the heck your gate is! The progressive taxi is fine but, I like something to align with. More high detail, or at least, properly sized airports. The default KBOS is FAR different than Grissom's scenery, which is right on.