Write it NOW! We MAY need it
This is very classic software management problem. Yes, it’s true that we may need it. But what if we don’t need it afterall ?
Software engineer is a bit different from other paper-work. We do similar work with the paper-work (typing, correcting, fill-up the form, etc). But the different is this. In paper work, the more that you type the less work that you will have. In software engineer, the more you type, the more work that you will get.
It’s true. Every piece of code that you’ve just added will add more complexity to your program. It means more time to review, test, and debugging (not to mention that any code that you’ve written will be kept forever as long as the product alive –backward compatibility requirement). At the end, clients will ask “how come making simple program takes so long?”.
My suggestion for this kind of problem is to be procrastrinate. Keep the request in your mind (or in your design sheet if you have), but don’t put it there. Not until someone really-really needs it.