Yup. Chicago is the natural rival assuming they want to go AFC/NFC with KC. Even then, Chicago is the bigger rival in baseball and there's no other sport that compares, although it trickles down some to college (kU/Mizzou) as well. Add in the Brewers/Packers and Wild/Vikings connection, and then a little bit of the old Red Wings/Lions, and NFC North makes the most sense. That said, I think if the NFL were to expand, it would add 4 teams (say, STL, Portland, San Antonio, Sacramento) and go to 6 divisions of 6, split 3/3 AFC/NFC and stop some of the splits between AFC/NFC.
Pacific (NFC): Seattle, Portland, Sacramento, San Fran., LAR, LAC
Mountain (AFC): Denver, LV Raiders, Dallas, Houston, San Antonio, Arizona
North (NFC): Kansas City, Detroit, Minnesota, Green Bay, St. Louis, Chicago
Southeast (NFC): New Orleans, Tampa, Miami, Jacksonville, Atlanta, Carolina
Midwest (AFC): Indianapolis, Tennessee, Cincinnati, Cleveland, Pittsburgh, Baltimore
East (AFC): New England, NYJ, NYG, Buffalo, Washington, Philadelphia
This would actually set up pretty well for a 16 game schedule.