Turning wildcards into other rarities / Rehash an oldshool MTG game.
I have 2 ideas. The first arose from never having enough rare wildcards to acquire the lands needed for most decks. It would be nice to be able to turn extra commons and uncommons into higher tier cards, or even split mythics into a couple of rares. It wouldn't even have to be a good exchange rate, but rather give players something to do with all their excess wildcards.
The second idea is more ambitious. Back in the 90's there was a MTG: Shandalar game that was fantastic. You started off with a 30 card deck and 8 or 10 life, and you quested around fighting monsters by dueling your decks against them, and winning more cards and doing quests to gain more life. You could also get them from dungeons or buy them from towns. I would pay good money for something like that for each new set.