My hands are not an issue when it comes to the cold and commuting but I'm considering some sort of bar mits for the fat bike. I ride with a lot of fat bike riders that use them (Pogies or similar for flat bars) and not necessarily for warmth. Because you can wear regular thin cycling gloves under them your contact points on the grips and controls feel normal which improves bike control and feel. This is a bonus for winter MTB riding. Heavy gloves, while plenty warm, can be clumsy and get wet and reduce your feel, plus what if you lose one fighting off a bear in the woods? Then it's a cold ride home.