My first take on a quick-and-dirty Magic: the Gathering RPG aimed at playing Planeswalkers would be a Nobilis conversion with the five colors as your ability scores. If I deemed the "land generates mana" mechanic of the card game something that should be reflected in the reality of the world, then I'd introduce some kind of mechanics where you need to ritually bind locations to achieve regeneration of miracle points (which would be renamed to Mana).