On the question of making your code flexible to anticipate future changes, XP is clear: don’t do it. Write the code for the requirements of the feature you are working on now, and if the requirements change, because of a new feature or user feedback, modify the code then. Since the code will have good unit tests, you can make these future modifications without worrying about accidentally breaking something because your understanding of the code is not fresh in your mind. And until you have new requirements, you won’t know what changes are needed, so it is foolish to attempt to anticipate them
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This of course fails as soon as you have "public" APIs -- that is, as soon as your code has callers whose code you can't see or test.
