What do you think about a new PVP-System? Every week there is a special ban that exclude a kingdom, element or race from the PVP-Field. If this happens there is no standard defense or offense that could be placed forever.
Another idea is to decrease the stats of cards with base rarity. For example: mystic cards have 50 % less life this week.
With all these limitations you can do new gamemodes with a few new difficulties.