Okay, on paper this idea might sound like fun, but I think it's obvious in practice that a league would flop. It's not effective and would be a hassle with little reward. Besides, PS has more important things to dedicate time and effort to.
Also, putting limitations on challenges would fail. If there was to be a league, it would have to follow standard rules of the tiers, not extra rules like "no legendaries." Legendaries are a major part of the metagame.
All in all, leagues don't work. A tip to all reading this: Even you can't make a league work.