Arma 3's weather is absolutely amazing, but many things are missing from having amazing weather in Arma 3. I have always dreamed of having amazing weather in Arma 3. With the inclusion of TrueSky, we now have a great head start into weather for Arma 3. Maybe one day we can have that perfect storm...
I link to Wikipedia as it is a quick way to get information and is mostly correct.

What needs to be improved to give a better immersion and compliment such a beautiful looking game:

Mirages:
In Arma 3 I always found the lack of any heat related effects like mirages on the environment very immersion breaking. Mirages can mess with your visibility on certain locations. Places in game like roads and the salt flats need mirages when it gets hot enough during the day time.

http://en.wikipedia.org/wiki/Mirage

Lightning:
Lightning in game right now is likely work in progress due to it missing sounds. But it currently only showcases one out of the normal three different types of lightning. The three types of lightning are: Cloud to ground (CG), Cloud to cloud (CC), and Intra-Cloud (IC). Lightning in game right now lacks variety as well. I am also pretty sure lighting has no affect on the ground were it strikes at the moment either. Lighting should strike metal metal material, trees, cause you issues if it strikes into the water near you, strike trees qnd give the trees a blackened model, and be a very rare hazard. http://en.wikipedia.org/wiki/Lightning

Clouds:
Right now we are severely limited in types of clouds in game. Meaning regular clouds are both rain cloud and regular clouds. With the Truesky technology, we need need more cloud types to make weather better in Arma 3. Clouds need to dynamically affect light levels on the ground and have shadows. We also need to ability to decide weather or not clouds can be rain clouds or not, because it is very annoying right now to have clouds forced into being rain clouds.
http://en.wikipedia.org/wiki/List_of_cloud_types

The Ocean:
Waves are ridiculously tiny at the moment. We need to be able to make them much larger, crash over at sea (Or white water), and look stormy. We also need a spray effect when a wave its an exposed rock in the ocean. Having waves crashing on the beach and potentially making ocean to shore and shore to ocean operations very hazardous. The visibility would be much lower as a result of an increased level of material in the water. Being able to operate boats in dangerous conditions should be comparable to helicopter in level of difficulty.

Rain:
Rain in Arma 3 has improved drastically in the development branch. What we lack now are rain effects on vehicle windshields that can reduce visibility unless windshield wipers are used. Wet and dripping surfaces while on the ground also help with the immersion of a wet and rainy environment. Rain can reduce visibility drastically and can reduce the quality of thermal imaging making a soldier job that much more difficult. We should also see rain water dripping off of vehicles and being blow off by wind and wind caused by movement like driving and flying.

Wind:
Not much to say about wind other than we lack a sound for it. Having it affect vehicles would be nice, though it would have to be only player operated vehicles as AI couldn't handle wind as we have found out. Wind gusts would turn a potentially easy situation into a very dangerous one. Wind direction would affect take off and landing with aircraft.

Traction loss:
With rain comes dangerous driving and landing conditions due to issues like hydroplaning/ Aquaplaning making travel on a rainy day potentially dangerous.

Aircraft should think twice about flying into large storms and instead fly around them. Weather also has to be synched between every player in multiplayer and cloud setting need to ability to be forced by the server/ mission like grass settings are, to prevent unfair advantages

If I missed any important aspects of weather, please say so below.