Baseball has the best chance of working. Hockey and Baskeball have confusing cap systems, revenue sharing etc etc and it would have no chance at all.
Relegation works in Europe because fans stick with their teams. Its a religion there. I remember when Fiorentina was relegated all the way to Serie C2 and they still kept their fans and will have a new stadium soon as well not to mention just recently fairing out pretty well in CL play. Stories like that never would happen in North American Sports. You have Yankee fans in Northern Ontario who really couldn't tell you right from left when it comes to NYC. Fans are fans in North America for different reasons. We as a whole really don't take sports as seriously.