Seattle, like Atlanta, also shares a stadium with an NFL team and also draws Atlanta-like crowds (I understand you're talking about relatively new stadiums).
Anyhow, I've only seen two MLS stadiums in person, in Frisco (FC Dallas) and in Harrison, NJ (Red Bulls). The Frisco one looks like it was built with expansion in mind. It's nice enough, but so basic you have to think they had planned to add on to it later. The Red Bulls' stadium is circular and has an overhang all around, so, while I'm no architect, I have to think it's as big as it'll ever be.
That being said, if MLS could draw at least 20,000 most of the time in most cities with a few outliers like Atlanta and Seattle, I would think they would be quite happy. Then again, the fact that they even awarded a franchise to Columbus in the first place (and that they allowed FC Dallas to be placed in Frisco, which is more or less a permanent handicap to that franchise) makes me think that, at least as of the mid '90s, they had no idea how big pro soccer could be in the US.