Q: As I looked at the new cards in Planar Chaos (which are awesome), I began to wonder what thought process the Creative Team used in relation to what creature types (Elves, Kavu, ect.) would go into the set. Do you just go with the popular types? Or do go by what types have not been used in a while?
--Bob Austin, Texas, USA
A: From Doug Beyer, Creative Team
That's a big question! I'm going into two main areas in my answer, but be aware that there are tons of corner cases and counterexamples that are too numerous for the scope of this question. Any particular creature type decision can have many motivating factors, so I'm just hitting the highlights here.
First, the creature types in a set depend on the decisions that go into the world-building for that setting. For example, it was important that the plane of Ravnica have the cosmopolitan, melting-pot feel of a vast city, and that it communicate a sense that organization and philosophy (i.e., guilds) had more importance for social categorization than race. So the creative team went wild with creature types in Ravnica Block -- humans, elves, spirits, viashino, vedalken, centaurs, elementals, gargoyles, ogres, griffins, sphinxes, vampires, frogs, weirds -- I could go on. The plane of Kamigawa, on the other hand, had one central conflict of kami (Spirits) vs. mortals (Humans and a few other types), so to reinforce this dichotomy, there number of different creature types in Kamigawa Block was relatively small. Sometimes races go away for a couple blocks and then return -- such as the re-emergence of loxodons in Ravnica: City of Guilds -- which helps preserve the coolness of those races and keeps players guessing about what will happen next. Time Spiral is set in post-apocalyptic Dominaria with the added feature of history-twisting time rifts, so just about any creature type from many earlier blocks is fair game for this block. Taken together, all these sorts of world-building decisions help give a setting its identity, and help ensure that any given world is distinguishable from the next.
Second, a set's selection of creature types depends on mechanical concerns. There are almost always a broad variety of sizes, colors, and abilities across a new set's creature base, so a rule of thumb is that a plane has to be able to support logically that kind of diversity with its creature types. Mirrodin Block, for example, is set on the plane of Mirrodin, home to humans (subtype Human), leonin (Cat), and loxodons (Elephant), among many other races. Smallish white creatures in that block tend to be leonin or human, whereas most larger white creatures are concepted as loxodons -- it's a simple example, but it shows how the creature types allowed for the mechanics of the set. Furthermore, mechanics that depend on creature type have special consideration. Creature types matter for the mechanics of cards like Goblin Warchief, Krovikan Mist, or Thelon of Havenwood; in cases like these, creative works with design and development to be sure that the intent of the mechanics is preserved while still jiving with the flavor of the setting.
So why is any particular creature type decision made? Why are there no Kavu cards, for example, in Mirrodin Block? The flavorful answer is simply that there aren't any kavu on the plane of Mirrodin. The mechanical answer is that there were no cards that required mechanical links or tribal interactions with Kavus, so their role of "medium-sized green or red creature" was filled by other creature types that were more compatible with the setting. There are as many answers as there are creature types, Bob, but broadly speaking, it always comes down to a combination of flavor and mechanics.