That wouldn't make sense considering the major league teams put their prospects in the minor leagues. For instance, what are you going to do if the Durham Bulls (the Rays' affiliate) end up being the best AAA team one year (they were the best last year), are you really going to promote Durham to the majors? What about their players, do they get to keep them? I don't imagine the Rays would be okay with that. And the rich teams will certainly end up paying revenue sharing to Durham for however long they're in the majors because they certainly don't have the market to support a MLB team. And if the Pirates got demoted and were forced to become the Rays' new affiliate, I don't think the Pirates' owners would be okay with being forced to make all their players available to the Rays maybe forcing the Pirates even further down the pole. No, there's just no way that would work.
You could, however,
create another league and relegate/promote teams to/from it.