I was originally going to make this a challenge to create cards that 'care' about the number 13 but the Innistrad spoilers spoiled that idea.
So instead this is all about luck. Wizards have used several mechanics to represent luck in the past - coin toss, dice, Clash, etc - you can use any or none of those, your card(s) just have to evoke the feeling of luck in some way.

My first entry (((Erratic Shapeshifter))) is meant to portray the random nature of Luck as it can be good, bad, great or terrible - all in the one game.

Woah, was't this something about D&D at first? I was up for that, I just didn't have the time at the time.

As you pointed out not everyone may knoe /play D&D so I killed that one until a later challenge and decided to play around with the fact that this was challenge 13.

Tricky challenge. I created Weighted Dice. There are probably a few other approaches to the problem, but none of them particularly easy.

Edit: I also created One-in-a-Million Chance.

­Chaos of Combat

I quite like Alex's too.

I made Morbid Game, though I'm not sure I like it. I think I'll revisit this challenge later.

­Heart of the Cards - attempting to look lucky without involving actual chance. Probably misses, though.

Added Rich Uncle Pennybags to the party. He's constantly concerned about being thrown back in the slammer.

Love One-in-a-Million Chance, Alex!

Mana Magnet Much better!!! (My last one sucked :P)