I'm not sure we would even need a retractable roof. Per this article on the new Vikings stadium, "Clear is the new retractable". http://fortune.com/2016/08/27/nfl-minnesota-vikings-stadium/
“Clear Is the New Retractable’
The Dallas-based architectural firm HKS Sports and Entertainment eschewed the retractable roofs used on some recent NFL fields. HKS found these roofs are opened on average just four to five times a year and cost some $75 million more to construct than a fixed roof.
Instead, HKS capped the stadium in a clear space-age plastic called ETFE (ethylene-tetra-fluoro-ethylene) to open up the field to natural light and the elements while keeping fans cozy.
“Clear is the new retractable,” quips Vikings Owner/President Mark Wilf in an interview. “I love the fact that you’re inside in a temperature controlled environment but feel the elements. You feel like you’re outdoors, you see the sky and the city and the weather.”
U.S. Bank Stadium marks the North American debut of ETFE, which was notably used at the Beijing Olympics Water Cube facility.
“The ETFE roof is the most unique feature, it’s probably the differentiator besides the iconic, super single frame,” says John Hutchings, who helped design the structure as principal in charge of the stadium HKS.
Hutchings says the ETFE allows sunlight to come through 60% of the total roof surface although it is filtered by small dots in the material known as fritting. He notes the material allows some heat to pass through and melt snow.
The roof has gutters to collect snow for a 100-year storm and Hutchings chuckles while saying he had “several lively discussions” with team ownership that it won’t collapse like the Metrodome’s fabric roof famously did after some large snowfalls.
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There's clearly no point in a retractable roof for American football in Minnesota, but you wouldn't do the same for baseball and I don't think you'd do the same for football. These are very different products, and I don't think this technology necessarily translates to other sports/outdoor venues.
*But I do think the stadium will feature as much natural light as possible, and change from older designs