That was apparently the challenge that Richard Garfield faced when...
It must be tough to make a good expansion. The proof is that there are a lot of mediocre ones, especially today when we have fewer “games” and more “core sets.” An expansion is essentially an attempt to catch lightning in a bottle a second (or third or fourth) time. A couple of games have pulled it off, but if a game has multiple expansions, it’s typically a case of diminishing returns. Either they ramp up the complexity, or they add more new content than any one group could possibly need. Add enough stuff, and a game could collapse under the weight of its own franchise.