When they added the 20,000 back in the 90s, they designed it so that another 20,000 seat bowl could be added when they felt it was needed.
HOWEVER, the upper bowl at the stadium has been crushing the original lower bowl. Note that season tickets cost so much because of the "Stadium Fee," which is going toward preventing the lower bowl from any further damage. In due time, though, ND Stadium will hold 100,000. There are some othercrucial problems though: hotel accommodations, parking, etc. One day, though, it will happen. I'd venture to say it will happen within the next 15-20 years. If you made me guess, I'd say 10.