The North American Union is a myth. No real agreements have been signed (so far, anyway). It is a terrifying thought though. I'm already disgusted at being ruled by a federal government that in general has no idea what would benefit or harm my area, and what my areas needs are. That would only get worse if you add more levels of crap filled people to the already astounding list.
As for NAFTA, I've never had a real problem with NAFTA itself. In fact, it's had a huge benefit for San Antonio. The Superhighway depends on which form you want to talk about. There's basically 3 forms I've seen.
One basically builds a limited access highway (parts of it possibly tolled) paralleling the Pan Am for the most part, with the idea of bypassing urban areas. This would be good for safety reasons, but would potentially hurt the local economy if it was easy to bypass SA to get to Dallas (bigger air terminal, etc).
The second is building a new interstate from the mexican border to Houston and then eventually up to meet with 35 in another state. This would certainly divert just about everything to Houston instead of San Antonio, since it has extremely large terminals for air, rail, truck, and sea shipping.
The last is just improving I-35 (even including some tolled cutoffs that bypass some metro areas). This is probably the best locally as SA would still be easily accessible, and it could potentially be cheaper to switch freight methods in SA to get to another major hub.
Once you figure out which form you want to look at, then there's pros and cons both locally, and nationally, etc.