Ian,
Part of the point of the new set is to allow exactly the customization you've mentioned. There will be 5 new colors of Good Dragons, 5 new colors of Evil dragons, and 10 new Mortals/WaitAndSee. The rulebook will have a number of suggested games, listing selections of dragons and mortals that create specific types of game.
So long as you play with equal numbers of good and evil dragon types, most mixes of the original and new cards will be fun. When I say 'most' I'm referring to the fact that there are a couple dynamics emerging in the cards that make a couple cards more interesting when played with specific other cards, not as interesting (or at least very different) when played alone. The rulebook will mention that...
I wish I could say more about the game now, but I just turned over the art order ten days ago, so the game is still in the production loop, and I've been asked to talk about the cool new aspects a bit later in the summer rather than now.
When we start saying more about it, I'll make sure to come back here and share the news.
--Rob Heinsoo
Game Designer at WotC
