First off: don't get me wrong, I love the design of the current gunslinger hats. The problem is there are only two designs, and there are six hats. So we need more. Obviously I'm not the only one who thinks so, I doubt it's actually escaped Three Rings' notice either, but for whatever reason they have yet to add more. So I might as well throw my name into the hat as long as concepts are being discussed.

Also, please excuse my artz. I can't actually draw, you see, and it took me so long to actually draw a convincing stetson that, by the time I actually realized what I had done didn't actually look like the hats in-game, I had already made several and was not prepared to start from scratch again.

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http://img837.imageshack.us/img837/3508/stetson.png

Gunslinger: The first hat in the gunslinger line. Currently a white hat with a gold sheriff star, the design here downgrades it to a simple brown leather stetson with a personal-color band. The band could potentially be left out for an even simpler design.

Sunset: The four-star version of the Gunslinger Hat, the Sunset Stetson gains a neat little silver star to denote a higher rank. I made the leather a bit darker to contrast with the sliver star, but it could also potentially be made lighter to be a sort of mid-ground between the darker three-star and the white five-star version.

Justifier: Uses the current design, which I drew for reference. The gold star is an upgrade from the Sunset's silver one, because even though the Justifier is not the only 5-star hat, it is the direct upgrade of the Sunset.

Nameless: Also keeps current design. It's drab and simple, which seems perfect for a hat with no name.

Deadshot: Not a stetson, but a gaucho hat (Mexican cowboy. Think Zorro, or better yet, Clint Eastwood in one of his Westerns) for variety. Colored black or dark gray with a white band to match the Deadshot Mantle.
* An alternate version of this has a white rim with little white pom-poms hanging down (or possibly tiny skulls if you can make them detailed enough). More festive, fancy, and admittedly a bit silly, but it's all about style, right? So long as it doesn't compromise performance.

Shadowsun: Brown leather that reddens around the crown like inflamed skin around a wound. Fiery fissures like those on the Vog Cub Coat erupt from under the band, representing the dark power contained within the hat.
* An alternate version has the cracks and redness along the edge instead of the base of the crown. My main concern is that the personal-color band may clash with the rest of the hat, so I just got rid of it and moved all the stuff somewhere else.
* A note: both designs are supposed to capture the Shadowsun set's evil power, as described by the Shadowsun Slicker's flavor text. However, the description of the Shadowsun Stetson, particularly the word "dashing," makes it seem more like something for a dark and brooding antihero than a diabolical villain. For that, the current hat design actually works fine, not to mention it does actually go really well with the armor, in which case the Nameless Hat would have to be redesigned instead.

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Actually, I do have one small issue with the current designs. I think the little breathing slits in the bandana over the mouth are kinda weird. I get that you're trying to combine it with the look of an armored helmet, but I don't think it's necessary, and stylistically it just doesn't work for me. The whole point of the bandana is to cover your face, which cutting slits kinda defeats the purpose of.

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Also check out Cobalt's awesome Deadshot Chapeau concept: http://forums.spiralknights.com/en/node/34203