I know it hasn't been officially announced yet, but there are so many leaks now, it's basically confirmed, and I can't keep my excitement quelled.
Here are some more leaked screenshots:
Looks like it will be set in a more tropical setting this time, perhaps back in Panau? Also, those tornados seem to hint at some dynamic weather, which should be cool. I hope they aren't just spontaneous traps that you can get caught in while parachuting. Rather, i'd prefer it if you could fly through them, and use them to slingshot yourself around. I'd love to have some battles in the air with a load of enemy helicopters while zooming between tornados or something.
Can't wait!