you should integrate Weather system (dynamic) snow and rain with snow surface shading and glossy rain shader , also good falling effect will be great like fs2004 and windshield effects or VC snow and rain effect
low visibility layers , 3D clouds (hd) and finally post effects (sun shraft , GI , reflection , rain halo's) HDR supporT AND DX11 , I cant wait demo
, please make demo flight simuator with all graphic suported , only map limited , Good luck !