It would be nice to have rain make impact sounds on certain surfaces eg ; Metal such as vehicles ,tin roofs, barrels, military HQ buildings ETC.
Metal is just an example but this is something you could get really creative with for the new expansion,This would be an amazing immersive feature.
You could also do wood, glass, different terrain surfaces, or perhaps interiors of vehicles could have a different sound when it rains.

You could make it the more intense the rain is the louder the SFX.

Thanks for reading