Have a look at Holacracy for interesting ways in how to structure teams and at the same time gearing up for continuous change. It's a very different approach than creating hierarchy, but it's not completely flat either.
I'd be careful with holacracy. There hasn't exactly been glowing successes in the press lately: 16% of Zappos's company quit over it, Buffer has completely abandoned it after a very public foray into it, and I saw a talk by the HR people at Medium and they've had to make significant adjustments to fit (for instance having one on ones even though there aren't technically managers).
http://www.holacracy.org/ and http://www.responsive.org/ for more interesting philosophies surrounding movements like Holacracy.
(We have a team of 10 people and are thinking about adopting Holacracy).