I agree with Deniz, nasty weather can have a big impact on the two most critical phases of flight, takeoff and landing. It's been so long since I've seen a good solid stratus layer in FS. In addition I'd like to see ATC that actually provides separation between aircraft. It would be nice if it could provide "services" to AI aircraft as well, but at the very least keep me from smacking into other planes while I'm on an approach in IMC. I imagine the day ATC gives altitude restrictions not randomly but to keep traffic clear of each other. Other commands would tell you or AI aircraft to speed up, slow down, hold at, etc. as required to avoid conflicts. It would also be nice if IFR flight operations could change based on weather. VFR conditions get visual approaches with traffic closer in. MVFR gets combination visual, contact (only if we request it), or instrument approaches based on the weather at the time. IFR conditions gets instrument approaches only, with the full separation, holds and delays that comes with it. LIFR could even force you to your alternate based on your equipment.