Some more possibilities...
Nairobi has a 28-floor convention center, admittedly hideous but it illustrates the point that we can go vertical too:
http://en.wikipedia.org/wiki/Kenyatt...ference_Centre
Cleveland's convention center is being expanded via private sector investment, i.e., the Hilton Convention Center Hotel. "The hotel will feature a 28-story tower filled with 600 guest rooms positioned atop a four-story podium of ballrooms, meeting space, retail space, and lobby. The hotel will feature a rooftop bar as well as underground connections to the Cleveland Convention Center and the Global Center for Health Innovation."
http://www.cuyahogacounty.us/en-US/C...l-Project.aspx
Maybe if Austin can let go of the idea that we must be able to attract the DNC or RNC (and face it, do we really want a bunch of liquored-up political a**holes running loose on Dirty Sixth?) then we could focus instead on expanding the number of less-ginormous meeting rooms and exhibit spaces, which would allow for a public-private partnership to build an expansion that goes vertical with multiple uses included.
The most profitable ACC event is SXSW, isn't it? That type of gathering requires many and varied spaces, not one enormous contiguous space. I just feel very skeptical about the need for that huge unimpeded floor space, as it seems to me that it severely limits the type of construction that can be used, while failing to attract enough added business to make it worthwhile.
What about highrise building(s) above the permimeter structural supports? On the roof of the 2nd or 3rd floor of the middle part of the convention center there could be a rooftop park/garden/outdoor cafe area for convention and hotel guests, surrounded by 2nd floor retail, restaurants, and shops. Are you picturing this? It would be very unusual and possibly visually unappealing from the street view, but I think it would be very cool and it would work from a structural perspective.