I've seen a myriad of reports about adding slings so that you can drop your gun at your chest and grab your handgun quickly, but I haven't seen any requests for actual slings.
Adding an actual sling to your rifle, instead of having it float there in front of you would be a really nice aesthetic feature. The same applies for large rocket launchers.
Perhaps if you wanted to get a bit more complex, you could have different slings, such as a two point or single point sling. It would be neat to toggle between letting your gun hang at your chest with a two point sling, or letting it hang off your shoulder, and for a single point sling, simply having it hang at your chest, or off to one of your sides.
The bottom line though is that if the gun is going to hang, at least add a visual for it so it's more believable. Plus, it's the little things that count.