Recent challenges:
Randomly choose a colour, converted mana cost, rarity, type, and archetype and design a card that fits.
Wizards said the cycle of Inscriptions is incomplete because they couldn't find a good design for the red or white ones. Let's prove them wrong.
Create a card that could be promoted as Christmas-themed, but could also be printed in a normal black-border set.
Read about Gwanhoz and come up with a top-down card design for the plane.