There’s a saying in writing for long-running series that you should only kill off a significant character if that character’s death creates more potential for stories than that character’s life.
Design space in games ripe for expansions should be treated in a similar way. For example, let’s say you make an ability that lets you cast a free spell once per game. Suddenly there’s a limit to the power level of spells you can create, no matter the cost you put on the spell. After all, this ability will just ignore the cost. This ability might be balanced if it saves you 5 mana, but broken if it lets you cast a more powerful spell. Suddenly you can’t reasonably design any spell that costs more than 5 mana. You’ve killed that design space.
As cool as it looks to let you cast a free spell, this price isn’t worth paying in most games. Some titles can get away with it, especially deckbuilders and other games where you can’t consistently assemble a killer combo. But that’s just a caveat to the example. We’ve stated that it would break the game if this ability lets players consistently save more than 5 mana, so you kill all design space that would result in that effect.
The Solution: Make a note of any ability/unit/spell/card in your design file that has synergy potential. Figure out how much value you’re okay with consistent and inconsistent synergies providing, then weigh your possible interactions against this guideline. When you make new content, consider how much design space that content is killing and if you’re okay with paying that price.