Best practices in other cities have shown that most successful new stadiums are located close to population centers and are convenient to both transportation arteries and effective public transit.
Where in Austin a stadium might be built is speculative at this point, but below are some sites that we believe should be considered
South I-35/Ben White
Looking south and perhaps with a nod to what was formerly one of Austin’s most popular outdoor music venues -- Southpark Meadows -- it has been suggested by a visitor to the site that the area adjacent to the I-35/Ben White/71 interchange be considered for a stadium. This area is close to downtown and has good automobile access. It is also adjacent to a disused rail line that has been considered for a rail linkage between ABIA and Cap Metro’s South Congress Transit Center.

Highland Mall

The 70-ish-acre site of the Mall fits the criteria of being convenient to downtown, near major roadways (I-35, U.S. 290 and 183, Airport Boulevard) and across the street from a station on the new Capital MetroRail line.
The size of the site also makes it attractive -- a small stadium would easily fit within the footprint of the existing mall. Nick Barbaro from the Chronicle put together a graphic that superimposes one of our example stadiums -- Skagerak Stadium in Norway -- on a Google Map image of the mall. The green line represents a restored waterway called Tannehill Branch, which is currently buried under the mall’s parking lot.

One problem with the site is that there is a shopping center on it. ;)
House Park
While not close to rail transit it does have proximity to downtown going for it. It also has an existing stadium on it. A small site and limited parking might make it difficult to enlarge beyond its current capacity of 6000 however.
The Aztex have a good relationship with AISD and the team is considering a move to House Park for the 2010 season, so it will be interesting to see what develops.

GRASS is not advocating any particular site and knows there are others out there.
If you have an opinion on any of these sites or have one that you think ought to be included in the discussion let us know.