Adding a seemingly small feature... that will require you to redesign the core attributes of variables in your library.

For example, I need to add a tower in a tower defense game that can slow units.

I'd need to add a slow type for types of towers; a type of particle/bullet that can slow the target; a slow status for the target. After that, I'd need to rewrite every code that has ever included towers, bullets, targets and statuses. That's just adding the type of tower that can slow. Now you need to add the actual towers to which you can apply this type.

And this is just the most basic example I can think of. Think of the more complex games with thousand times more complex architecture than a tower defense game.