Take Cosmic_Encounter?. Separate out all the Flare and Edict cards. Make all the Challenge Cards into characters or equipment or events complete with pictures and history, and mix them in. Print large numbers of this collection of cards but make sure you print more of the less powerful ones. Sell them in groups just big enough to play a game with, and in little packets guaranteed to have at least one pretty good card in them. Profit.
This basically describes how Collectible Card Games (also known as Customizable Card Games or Trading Card Games) work. The people who buy the cards can play with just a few, but they won't have access to the really good ones unless they buy a lot of the little packets. The games are designed so that certain powerful cards can usually overcome other cards, though wily players will often find ways to exploit the rules with (cheaper) common cards. Because the game is based around a core set of rules, with large numbers of cards that modify those rules and each other, loopholes arise with a kind of inevitability. There's probably a mathematical law that describes the phenomenon. At any rate, the loopholes require the creation of errata to clarify things and make them fair again. Then the company produces more cards and the cycle begins again.
I haven't done any research on this, but I'd wager that CCGs are the most popular form of non-video game in the country today (Valentine's Day 2003). If a person don't actively play one of them, it's a good bet that he or she knows someone who does. Regrettably, this has little to do with how well the games play, and more to do with their marketing tie-ins and money-soaking format I mentioned above.
Think of a popular science-fiction or fantasy property and it's a good bet there's a CCG made for it. Star Wars, Star Trek, Babylon V, Dune, X-Files, Tomb Raider, Harry Potter, just to name a few, have all spawned CCGs of varying quality. Others, most notably Pokemon, started off as CCGs and have spawned in the other direction, with spin-off TV shows and movies. I will freely admit that my attraction to the Star Wars CCG was purely due to the name. If it had been called "Space Wars" or anything else, I'd never have given it a second glance.