Some way of slinging your shotgun on your shoulder is a worthwhile thought for the waterfowler. I don't think I've ever met a waterfowl hunter who carried too little stuff to the blind and couldn't use an extra hand. Today many American shotguns are easily equipped with after-market sling swivels, and some come that way from the factory. This was not always so. The continental Europeans have always been fond of slings on their shotguns, but slings, and a method of attaching them, have been uncommon on American shotguns until recently.
Not all shotguns are easily equipped with sling swivels and a sling. For such guns, a case with a sling is the answer. There are some very good ones on the market that are also thick and protective, will float (with your gun inside), and come in popular waterfowling camouflage patterns. For really nice guns that you don't want to drill holes into (to install sling swivels) and also want to protect from the rigors of waterfowl hunting, these are a good deal.
I guess the final "fine-touch" shooting accessory would be the shooting bag. Back when muzzle-loading hunters carried unassembled ammo and a plethora of shooting gadgets to the blind, the shooting bag was an absolute necessity. Today it is merely immensely convenient. Not only does the bag carry your extra ammo, it serves as convenient storage. The shooting bag should be the place where your calls, shooting glasses, ear plugs, and other necessities "live" between hunting trips. Having a well-organized shooting bag means never having to say, "I forgot..."