I have this crazy theory that "Plan 5," which had the Cowboys and Cardinals in the Central while the Vikings and Lions went to the East, would have ultimately resulted in the Saints moving to the AFC.
The only reason the Cowboys remained in the NFC East after the 2002 realignment was to preserve their storied rivalries with NYG, Philadelphia and Washington. Geographically, it made little sense, but what did make sense was to keep the team with the largest national fan base in a division with three of the nation's largest media markets.
Had they begun in the Central, however, Cowboys-Packers would have been an instant rivalry given the fact that both teams had been dominant for several years prior to the 1970 season and had met in multiple high-profile games. But aside from a few hotly contested races early on, Dallas would have likely dominated the division throughout that decade much like they did the NFC East, due mostly to Green Bay's decline after the departure of Bart Starr. The Bears would have gone on to establish the dynasty they achieved in the 80s, followed by a 90s revival of the Dallas-Green Bay rivalry as both teams rose to prominence under HOF quarterbacks.
By the time Houston entered the league in 2002, it would have been much more difficult to keep the league's marquee rivalries intact while drawing divisions along geographical lines. Keep in mind, both the AFCS and NFCS as we know them were initially composed of teams who generally did not have long track records of success when compared to other franchises. At the time, none of the eight (ATL, CAR, NO, TB, IND, HOU, TEN, JAX) had ever won a Super Bowl while playing for their current host cities - the Colts held the group's lone Lombardi Trophy, which was won in 1971 when the team still played in Baltimore.
While it's somewhat feasible to consider Dallas an eastern team, it would have made absolutely no sense to place them in a newly-formed NFC North. At the same time, if you were the NFL, to break up a premier rivalry that ran from Starr-Staubach to Aikman-Favre would be pure lunacy. So what was the NFL to do?
I believe they would have seen a rare opportunity to preserve all of the league's most historic rivalries while at the same time capitalizing on the most commercially successful ones of the preceding decade. With minimal impact on historical ties, the NFL could have at that time revamped the NFC West to include the Cowboys, Packers, Rams and 49ers.
The NFC East would encompassed the Giants, Redskins, Eagles and Vikings, who would be forever connected by a period of titanic competition from 1988-1992, in which the four teams combined to average 40 wins per season.
The NFC North would have been a regional marketing dream, encompassing four teams in Detroit, Chicago, Carolina and Washington that resided within the same (EST) time zone.
With only the NFC South remaining, there would have been six obvious candidates to populate the new division: The NFC's Saints, Falcons and Buccaneers, the AFC's Jaguars and Dolphins, and the newly formed Texans. Right off the bat, Houston would have been a non-starter, as it's widely believed Jerry Jones wanted nothing to do with second Texas team within the conference. Meanwhile, the other southern franchises were dealing with their own detrimental situations. New Orleans was suffering from a fan exodus that (according to some sources) left more Cowboys fans than Saints fans in the state of Louisiana. While that trifecta of teams battled for positioning in the football-crazed southwest, however, the three Florida teams struggled just to fill their respective stadiums on game day.
Enter a solution that could have worked to the benefit of everyone. By assigning both the Saints and Texans to the AFC South, the NFL would establish a geographical rivalry without pitting either franchise directly against the Cowboys, thereby allowing fans to follow both teams that enjoyed widespread popularity in their respective regions without conflict, similar to the relationship between the Giants and Jets in New York.
To round out the conference, the league would cluster together four teams (Atlanta, Jacksonville, Tampa Bay, Miami) in a geographic area from which it historically struggled to attract viewers. The subtle brilliance behind this configuration, however, is that this version of the NFC South would serve as a near-perfect analogue to some of the most prominent rivalries in college football, which happens to be wildly popular in the region. It's not hard to imagine hardcore fans of the Bulldogs, Seminoles, Gators and Hurricanes aligning themselves with the Falcons, Jaguars, Bucs and Dolphins, respectively as extensions of their already existing rivalries.