The "vision thing" is much derided, but if a program will be doomed for lack of organization it will not even be born for lack of vision. That some Sunday School programs continue to function without vision is not a positive.
Decide what adult Sunday School is for. Should it be a small group environment? If so, classes should not exceed 12 students.
If a church has a small group program already, then Sunday School cannot serve as a small group program as well without redundancy. In these instances, Sunday School should function as an education environment.
The difference is where small groups focus on relationships first and education second, the Sunday School environment can invert the priorities. Pastors who recognize the value of relationships in discipleship may dismiss this idea out of hand, but they should not be so quick to do so.
Not all aspects of the church can serve all the church's needs. For example, we would not expect a children's program to retool to appeal to adults. In the same way, we should not expect an adult Sunday School program to function like the youth ministry.
A solid education program can be a huge asset to the church, but the leader of said program must decide that is what they want it to be.
Once the vision is decided, the pastor or leadership team needs to get behind it in a public way. Announce before each term what classes will be held and why students should be there.