They could to focus on something new on league to make players bring these stuff to their non league character at the end. Example d3 focused on paragon. What is a wrong choice. You could focus on making player renew all piece of gear everytime with better gear.What is what mmorpg should be based on: hunting/grinding/finding new itens.

Example:

After player reach level 70 all itens dropped will have additional extra affix + 50 from a random attribute (wisdom, agility, ferocity, toughness). This way the players will need to renew all their set.

After player reach level 75 all itens dropped will have additional extra affix + 100 from a random attribute (wisdom, agility, ferocity, toughness).

After player reach level 80 all itens dropped will have additional extra affix + 150 from a random attribute. (wisdom, agility, ferocity, toughness).

After player reach level 85 all itens dropped will have additional extra affix + 200 from a random attribute. (wisdom, agility, ferocity, toughness).

After player reach level 90 all itens dropped will have additional extra affix + 250 from a random attribute. (wisdom, agility, ferocity, toughness).

And additionally to that they could to introduce 2 new active skills for each class (melee, ranged, spell) cuz if every league you force players to run bleeding edge or sword spin it will be sick. This way you will renew the play style and players who wanna new skills they will play league to take them to their non league character.

And to finally end that with golden key increase one additional wheel of fate circle to league so each character hits at least level 80 on league they will be enabled to bring the new wheel of fate to non league character.

And league should be a better place to farm gear than non league so add one additional affix for every item with a rule - this affix can repeat other affix the item already has. This way a lot of combo could to show up with a lot of crafting stuff. So players would have a great reason and motive to farm/profit a lot of itens on it.

Strategies afterwards from first league - 1 - introduce each new league a new end-game weapon, ring, amulet, shoulder, belt.
2 - Estimulate players to level all active skills (all those not used - for that you have to give a little bonus for each not used skill leveled - this way player will take a lot of time to level all skills to level 70).

This way i would play league - it would seem more atractive.