An important element in card games like Magic: the Gathering and Race for the Galaxy (and probably many others) is the one of synergy. Find cards that work together and build your strategy on that. How well the individual cards combine is the main success factor in the game.
How do you build something like that? Something that I'm thinking about is:
- Define some strategies that lead to victory.
- Create a bunch of cards / game elements for each of those strategies.
- Playtest like hell, to find a good balance.
Is that the formula? Are there other ways to do this?