Promotion and relegation, which is done in foreign soccer leagues (but not in North America's "Major League Soccer"), couldn't be done in the U.S. Not because of the vast distances involved (the Jet Age renders that a bad argument), but because of the ties of minor league baseball, basketball and hockey teams to major league clubs, as part of their "farm systems."
But if it was done, here would be the relegations and promotions for Major League Baseball for the 2011-12 off-season:
American League East: Relegated, Baltimore Orioles; Promoted, Patwucket Red Sox. As bad as their parent club finished this year, the Triple-A team, outside Providence, gives their fans some hope for the near future.
American League Central: Relegated, Minnesota Twins, after winning the Division 6 of the preceding 9 years. Promoted, Omaha Storm Chasers, formerly the Omaha Royals of the American Association, now Champions of... the Pacific Coast League? Since when is Nebraska on the Pacific Coast? Apparently, since the American Association folded and its teams were merged into the PCL.
American League West: Relegated, Seattle Mariners; Promoted, Sacramento River Cats.
National League East: Relegated, Florida (now Miami) Marlins; Promoted, Durham Bulls.
National League Central: Relegated, Houston Astros. Promoted, Columbus Clippers, former top farm team of the Yankees, now (much better geographically) of the Cleveland Indians. They won this year's Pennant in the International League.
National League West: Relegated, San Diego Padres. Promoted, Round Rock Express, the team outside Austin, Texas, owned by Nolan Ryan and two of his sons (and sort-of named for Nolan).
Of the 7 teams to finish first in a Division of a Triple-A league, the Reno Aces had the lowest winning percentage, and are thus the odd team out, for that reason, rather than for geographic reasons. I suppose a Playoff between the 6th and 7th seeds could be held -- or, perhaps, one-game Playoffs between 4th and 7th, and 5th and 6th, and the winner would then be the new 4th seed.