I saw some "skinny" 20" barrels for sale the other day. The first thing that popped into my head was a retro M16A1 build. Then I remembered why all my builds are flat tops - can't see the front sight anymore without glasses.
I guess the only "retro" looking AR15 I'd really be interested in is an XM177 style gun. Mine is on an A2 upper so it doesn't really look "retro" with the big adjustable rear sight on it. It does look kind of cool with the Single Point sight on it though, I just don't have the right rings for it, but it still looks cool.