No matter how well designed and elaborate your application is, there is always more functionality that the user wants from it. Of course, with a proper framework in place and your code well documented, changes should not be a big problem, but on the other hand, you wouldn't want to restart an application just because some minor customizations are needed.

Many requests regarding an application will be concerned with the usability of the user interface, for example, different behavior of widgets or some additional features with regard to the information shown, like spell-checking entered text, finding stock market information related to a company shown, or the current exchange rate of a value on screen in a different currency. Quite a few companies including Microsoft, Yahoo!, and Google offer all sorts of free API's that may be used to enhance the display of values.