What they could do is have an east coast city National in the spring and a west coast city National in the fall (or vice versa). And, then have a midwest coast city (probably Chicago and/or Cleveland) replace one of the coast Nationals the following year like so...
Spring 2011 East coast city National
Fall 2011 West coast city National
Spring 2012 Midwest coast city National (to replace the East)
Fall 2012 West coast city National
Spring 2013 East coast city National
Fall 2013 Midwest coast city National (to replace the West)
and so on.....
