I'm designing a card game that I'm implementing on the computer that mirrors some gameplay aspects of dungeon crawlers and RPGs. The player's object is to survive as long as possible from the onslaught of monsters, etc. in the dungeon, while getting stronger and more powerful. The way the dungeon is generated is by drawing one card from the "dungeon" stack per turn, this card represents a monster, animal, item, etc. found in the dungeon. As the player progresses through the game, he gains better abilities to fight the monsters and animals with. The issue becomes that the player then becomes too powerful to have any challenge (e.g. after progresses through the game for a while, the rats in the dungeon won't be a challenge). My question is, how can the player be challenged, assuming that any cards in the "dungeon" stack cannot be removed, and that the player must become stronger (e.g. not progressing is not an option)?
One solution I thought of would be a leveling system, where the player's progress is measured through levels, and upon attaining another level, certain, more challenging cards are mixed into the "dungeon" stack. This however would involve different card backs (theoretically not an issue in the digital implementation) which would give away the difficulty of the card the player is about to draw.