What about him is that much unlike the others?
Besides that, if the writers and design staff stuck to the same thing every year for what a "proper 6th" should be, then the series would get way too formulaic and it'd get boring. Making something different from normal every now and then helps to keep it fun and enjoyable.
Other than that, while Gold might be impressive and Shiny, it's not like he doesn't bring something to the team or the story. He's a power outside the "norm" of samurai in the story and in a sense represents how people became samurai during Sengoku Jidai. 18 alone though is probably one of the first chances he's gotten to really cooperate with the others so I don't really know how you can accuse him of not being a part of the team either. He's barely had a chance to really work with the core 5 and he represents a "normal person" outside of the Kuroko dynamic. The upcoming episodes are going to be showing more of that, so I think you might be jumping the gun a little too early.
Gold does have a lot of flash and humor to him, but he has a genuine desire to want to be a part of the team and as a part of the story element, this'll show some interesting conflict with Ryuu since he's very by the book and by tradition and lineage. Despite all that he *wants* to do to help Takeru though, it's only been one episode and one Gedoushuu where he's been able to really do much about it. 17 was an intro where he kind of came out of nowhere and he was more "separate", but 18 is where he officially joins and while he's worked with Takeru in 18, they also have a background history together so it's more natural. The upcoming eps we get to see his relationship develop with everyone else. After that, pass your judgment on whether or not he's all flash.