I know this has kind of already been suggested, but how about a card mixer?
It takes two cards, and, well, mixes them. It would choose what the card does by what you put in, for example:
If you were to put in a Lava Golem, and a Dragonfly, it would have a casting cost between 1 and 4, an attack power between 1 and 5, health of either 1 or two, and either no ability or the growth ability. It would also be cast with either Fire or Air.
Boring?
Well, the fun happens when you combine different types of card. For example, a Phase Dragon and a Miracle. You could have an Immortal creature with the ability to cast Miracle! Or you could have a Phase Dragon, and a wasted Miracle, because it would have a chance of combining that is proportional to the power of the card.
A Phase Dragon with Miracle could perhaps only appear once in every 20 tries. That's a lot of Miracles.
Everything is determined by power. A combination of Lava Golem and Dragonfly would have a higher chance of not getting the ability that getting it.
A card that is immortal lowers the chance of good combos, or may even lose its immortality. Pillars, Permanents, Spells, and creatures can all be combined.
Pictures would be fairly easy, a mix of fire and air would have a picture of half of the fire symbol and half of the air symbol. If you were to combine that with entropy, then it would be 1/3 fire, 1/3 air, 1/3 entropy, etc.
It would probably be hellish to code, though.