Tileset switching (eg. snow results in snowy tiles)
Weather modifiers: If we wanted levels on Earth let's say, then Rain might result in 0.5x speed modifier for ground units due to mud, Thunderstorm might result in air units grounded (probably I'm going too far off base with that) Dust storms could result in lowered visibility / ineffective radars.
graphic, movement, layering and anything else that may come to mind [wind/wind speed?]