I think it would be cool to simulate severe weather based on current or predicted SIGMET and AIRMET reports.
They are public and can be found easily on the net. They comprise of a “coded” message that states current or predicted severe weather conditions and their localization.
Several phenomena can be reported or predicted by SIGMETs and AIRMETs.
I would leave things like thunderstorms to the sim and its weather engine alone, however reports of thunderstorms with hail are sometimes reported and the hail could be simulated if a storm is simulated in the game as well.
I would mostly use it to predict moderate (AIRMET) and severe (SIGMET) turbulence and icing (that’s turbulence and icing not connected to severe convection, you already expect those), severe and moderate mountain waves, dust and sand storms, volcanic ash clouds etc.
I think it would add a big degree of realism to the sim, not only because you could properly use weather planning for your routes, but also because the info that would be fed into the sim could be properly displayed on weather radars for example.