Some time ago they had a contest for fans to design accessories and the winners were added into the game. This is why a lot of the unique items have flavor text that tell a story, or are referential, like Kitty's Collar.
As far as the sigils go, well it's all about using the right sigil for the right class or character, in the right weapon. Rogues are not supposed to get hit; they have abilities that allow them to avoid damage, reduce their threat, and so on, in addition to your tank hopefully doing their job. A sigil that gives the rogue reduced damage resistance along with some other powerful bonus will only really be harmful is the rogue is played poorly. I don't think sigils are for all players. I don't use them in my own games. But they are a fun way to further customize gameplay.