One of the results that most surprised me was how little each of my themes and mechanics became apparent during play. I believe MaRo has said that "it's not a theme if it's not on a bunch of common cards", and that makes sense, but going in to the playtest I had thought that (for example) Resonate was on a bunch of cards. It turns out that there were way too many (common) cards that referred to or were affected by resonate, and not nearly enough that actually had resonate. This was pretty much true for my other two big themes as well: Debris (lands being relevant even in the graveyard); and Husks (piles of 1/1 creature tokens that are overrunning the land.)
What I learned from this is that I need to be more deliberate in putting the set together. For example, there needs to be a structure that dictates how many common white enchantments are in the set; how expensive they are, what rarity, and how many of them should refer to at least one of the set's themes.
For now, I'll start with figuring out the breakdown by rarity, and get into more detail about card types later. I just checked Mirrodin and Champions of Kamigawa, and they have:
286 cards (not incl. basic land)
110 Common : 88 Uncommon : 88 Rare (actually Kamigawa only has 87 uncommons)
So, the same number of uncommons and rares, but 125% as many commons as either of those.
In my set, I'm aiming for the following structure:
265 cards (not incl. basic land)
115c : 85u : 65r
Almost 175% as many commons as rares! What am I doing?? Well, I think WotC includes the amount they do in a set to encourage us to buy more packs: the more rares, the more incentive to buy packs since you only get one rare per pack. (Of course, by this logic, they should have 500 or 1000 rares per set, so that you have to buy thousands of packs if you're trying to complete a set or get 4 of a given rare. But WotC also has to consider the willingness of the players to spend, and I'd bet that they've arrived at these counts thru market research and trial & error.)
My emphasis is not on selling packs; it's on having fun drafts. I want my small group of players to see a good variety of common cards and have a reasonable chance of seeing most of the rares over the course of a few drafts. I don't know how willing the guys will be to keep testing my set - hopefully they love it and we play it a bunch, but it might not gel immediately and they might run out of enthusiasm for testing...At WotC, they can iterate on a design until it's good, because their testers are paid to be there trying things out.
I should say, I didn't pull those numbers out of thin air. Based on what I've got so far, I've decided to go for 47 cards per color, plus 18 artifacts and 12 lands.
47 * 5 = 235
235 + 18 + 12 = 265 cards in the set.
I also will be adding 20 multicolor cards, bringing the total up to 285 cards, but I don't know their final rarities yet.
I think I'll stop there for now, but I'll come back to talk about how I have "slotted" the card types across each color in my set.
Today's Preview Card (subject to continuing development, but this one is pretty popular and not likely to change).