This is a problem I found generally in collectible and non collectible card games. But other type of games can also have this problem.
When a game is good, some times they make expansions to the game. But these expansions where not planned when the game was first released. Now they continue to add expansion which are cumulative with the previous ones. Result: the games is much more complex, completely unbalanced and the look and feel of the game is totally different. So by seing it this way, over time, expansions makes the game worst.
The best example is MTG(or course). It now has more 20000 cards and you all know that there are many esoteric combos possible. (ex: in a game I played recently, each time a monster was played, 64 copies of this monster appeared on the table and all enchantments became monsters(with also 64 copies). So There was more that 800 monster on the table summoned in 1 turn)
This phenomemon is mostly due to the fact that an expansion must always do more that what was done before. Else, there would be no need to really buy the expansion. So when your obsessed with the need to create more new things every time, the total game balance takes a hit.
Sometimes, you can plan in advance some expansions. For example, in duel master, they already planned that there would be "evolution" type of creatures in future expansions, so they talked about it rule book.
For a game like munchkins, there is a standard balance that make sure that each theme pack has the same number of class, level up, wandering monsters, etc. But it get abusive when some element does not replace each others. For example, in munchkin, classes from any game over lap each other, but races and styles(munchkin-fu) does not. So you can have a character with a class, race AND style. So this is where it start to become more complex and unbalanced.
A example of good expansion would be the two settlers expansion(knights and islands). They add complexity, but it is still playable. But most of all, these 2 expansions are not cumulative so keeping control on the game is easier.
So is there a ways to make expansions without unbalancing the game on the long term.?
Here is the list of possible expansion types I have seen so far, feel free to complete :
New permutation : Consist in making new cards that has permutation of stats or abilities not used in the original game. No new abilities or rules are available. The problem is that there is no innovation and you can run out of permutations.
New Stuff : This is the general expansion type. Add new abilities and rules. But on the long term it can unbalance the game.
More components: The best example is the expansion for 5-6 player for settlers. It does not change the game at all, it only gives more components to play with more players.
New Scenario: In a game where there is adventues or scenarios like dungeon crawlers and war games, you supply a list of new setups, dungeons and sometime new items. But in general, the rules stay the same. It only increase the playing time.
Totally new game : This is the idea of making a new game using the same components. You create a totally new game that has nothing to do with the first one, but the original component can still be used. For example, in the PS2 yu-gi-oh, you place monster on a grid and you move then on the board. Your character(dragon) is on the board and you must kill the opponent's character. The summoning is totally different, it now use a point system(according to monster level).
