I`d like to suggest new Dungeon levels where players could find gems, perls, gold items (not ordinary ones, because we have completed Dungeon I).
It shouldn`t be hard for you.
If you want to make it harder, copy bases of top 100 players.
If you want to make it more interesting, relate completition of Dungeon II certain levels to upgrate of troops or towers (same as for beginners, they can reach new spell only when they upgrade throne).
If you want to make it the same difficulty for player No.1000 and player No.1, you could ban using scrolls in Dungeon II. Like this even top players could struggle in last chambres of Dungeon II.
Uber item upgrades are very expensive in higher levels and success rate is low, so you could put more perls in Dungeon II and have more often special days when perls are multiplied by 5 or 10. Or…when somebody succeed in a certain Dungeon chamber, it will unlock all Blacksmith meltdown for a certain number of days.
I don`t like items without any perks (I have one like that, where You have forgotten to give a perk at the beginning. I just hope I could have 2 special perks on it