You could pass the hot potato to someone else. You could tell the client to go jump off a cliff. You could be super-polite, promise to think about it, and hope the client forgets. The client never forgets. Small enhancements that screw up major functionality.
Additional data flows.
There hasn't been a project since the beginning of recorded time that hasn't been faced with such things. Napoleon wasn't satisfied with Western Europe. He had to have Russia, too. It is a natural phenomenon. As the product takes shape, the vistas of possibility grow wider for the client.