Quests that only use certian star champs.
With trying to keep the lower star champs reliant and keeping people leveling and ranking less star champs... Y not have quests that only u may use 2 star champs?  Only 3 star champs?  Have rewards for those beginners... But get much harder and offer good rewards for those skilled players..   Just a thought but I think it'd better than bases... Or some of the other stuff u put into the game.