Day/Night cycles
This has been mentioned by Sp3ctrr and others. Simply a Random global Day/Night cycle for each map, and if it's night mechs would have lights and stuff like that.
Rain/Snow
Pretty self explanatory, good for showing off PhysX
Fog/Low visibility
It would make it wayyyy harder for scouts and other A-class mechs to just run around without getting themselves into a bad position, which I think would make the game more balanced. But maybe that's just me.
Wind
The game already has somewhat destructible paraphernalia around the map, so wind effects are just one step away.
I don't know much code, but I don't think it would be that difficult to add these effects, and it adds a new spin on the game that will hold us over until the next major update!