Maybe they are all blonde?
I doubt they have any significant role, much like the Hatake clan or the Haruno clan. They'd have been introduced earlier if they were to become a large part of the story.
Minato was the only Namikaze worth mentioning, as Jiraiya referred to him as a 'once in a lifetime genius'. He seemed to be a regular shinobi, albeit with better stats than most, until he acquired FTG and Rasengan. I'd guess that any other Namikaze would have been fairly basic as well.