Some applications seem to be finished. They do everything that is conceivably possibly. Consider Excel, for example. Is there anything that’s not yet possible but should be? Or imagine a calendar application. Everything is already developed, after all, what could there be added?
Still, there are some enhancements that can be made, mainly by linking the original purposes to related purposes. This post on Little Big Details shows how Outlook displays the weather for each day directly in the schedule. Originally two application, the calendar and the weather application where separate, but the integration saves a lot time and effort in seeing whether the weather fits to a planned activity.
I conclude that for such finished applications, integration with other applications can still enhance the product.