An important question to ask when designing a set is what world is it set on? I assume it will be a custom world which gives a lot to the world planning. What do we what as the man races on the world? Humans, elves, merfolk, vampires, giants, minatours, treefolk, angels, ect, ect? How do we want to split up the mana pie? All worlds have all 5 colors of magic unless there some seriously wrong. Will we have a nations aligned with 2 maybe 3 colors? Once we have a idea of what kind of fantasy setting we have, it would be easier to create a story and easier to lay card mechanics with the lore.