Here are some of my favorites...maybe we see some of them with the next expansion...
Thieves Guild armor (Skyrim): While there are some armors that are thieve themed, none of them really looks nearly as good as the thieves guild armor - or even like an armor a thief would wear (with lots of pouches, bags for tools etc.)
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Dawnguard Armor (Skyrim): One of my all time favorite armor sets in any TES game
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Storm Cloak Officer armor (Skyrim) / Bear armor (Morrowind)
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Madness Armor (Oblivion)
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Ancient Nord Armor (Skyrim): While there is draugr armor that is slightly similar, this looks SO much better
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Dragon Scale Armor (Skyrim): We got the dragonbone armor outfit but it looks SO bad compared to skyrims dragon armor - so i hope we might get another dragon themed outfit or armor; and the shield looks amazing!
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Vampire Armor (Skyrim); i think you could get quite creative for a vampire armor set (or outfit)
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Wolf Armor (Skyrim), another favorite