I always liked the idea of introducing a game mode where you can fight monsters in a progressive survival format, while being restricted to using just one element (this encourages collecting).
Instead of a dungeon, why not a vertical tower? Each time you beat a set of opponents, you move up the tower to reach your next fight. Unlike normal battles, however, this is survival mode, so a killed monster remains killed and cannot be revived even with gems.
There should be a Tower for each element (maybe 15-20 levels each tower using the catalogue of existing monsters), available to play each day. What changes each day is the element restriction for each tower. So one day you can aim to scale the Dark Tower with all water monsters - yet the next day you can only use metal monsters in the Dark Tower, etc..
It would be nice to amass an army of Thunder troops to try and scale atop the Water Tower in between Team Wars and PvP.
Rewards for this Tower Survival Mode would be badges (bronze, silver, gold) to prove how you are at attacking each tower with each element. Much like collecting monsters in the Library, collecting a full set of badges for each element's Tower also gives you rewards.
And much like The Keeper's association with books, I think The Warrior should be assigned to the badges.