As early as the end of 1969, Tex Maule was writing in Sports Illustrated that "Although there are no immediate plans for expansion, the NFL will, eventually, mushroom to 32 teams" and Tex Schramm was forecasting something close to what would eventually happen, albeit over more than three decades ("We have nothing to gain by adding one, two, three or anything less than six teams. The next logical change is from our present setup to 32 teams, split into eight four-team divisions, four divisions in each of two leagues, two divisions to a conference. That would lead to logical playoffs between divisions, conferences and leagues.")
Besides envisioning that the NFL and AFL would still be separate leagues, each split into two conferences, Schramm also had the idea of doing something that the NHL had done with ice hockey a couple of years earlier, which was to add six expansion teams at the same time, after letting them first play in their own division before being integrated into the 26 existing teams. He was right about Seattle being an expansion team, but didn't get what he would have liked to have for the other five. From what I gather, he expected that this would be accomplished by the end of the 70s.