Well ROOST already affects certain weapons. Specifically, it affects the MG barrel cool-down time. The colder the ambient temp is set to by the mapper, the faster it cools down.
Other than that, I don't know of any other effects.
If they really wanted, they could model the ballistic effects of colder/denser temps on bullet trajectory, but I'd think it would be negligible.
I do recall suggestions on the ROOST forums for visible breath when it's cold. That would be kind of cool, and very atmospheric.
As far as effects on players, I don't know about grave robbing clothes, but I wouldn't be oppose to modeling one side being better equipped with cold weather gear. Warmer clothes would lead to more stamina (it's really tiring to be freezing, as your body uses a lot of calories to keep you warm). But the downside might be movement speed, as you would have bulkier / heavier clothes. If the mapper really wanted to go crazy, they could assign warm values to indoor areas, and cold values to outdoor areas.
That might be beyond the scope of the game though.