Have the ability to change weather from Sunny, to cloudy, to raining, thunderstorm, after-rain, clear, snow/winter, frosty, foggy etc etc. Yes it's already possible to enable fog, change time, removes clouds or add clouds, add snow or rain, but it would be nice to have that simple feature to adjust the weather rather than opening World Editor. Then therefore the weather impacts the driving abilities on roads and road conditions.
Honestly the idea is pretty cool, an easy access and adjustable weather. But personally, I think for the "affected" road because of the weather is kinda difficult to do. However, seeing that there's an UI app that lets you change the "grippiness" of the road in real time, it might be doable to connect both "weather code" with the one that changes the properties of the road. But Im no modder anytime or soon, so Im not really sure about that.
There's also wet physics, as seen on ETK driver experience center and gridmap. That would require a bit of work to generate it on the roads but it's doable.