Every iOS application should be able to define types of input and output data. For Tumblr, an input type could be a photo or a video to post, while an output type could be a credential that another application could use to communicate with the Tumblr API. For 1Password, an input could be a URL, and the output a corresponding username and password. Allowing applications to be stitched together dynamically based on the data types that they know how to act upon would allow iOS users to be productive in ways that are not possible today.
Apple’s strategy of implementing just one or two third party services into the OS every year isn’t scalable. It works great and provides exactly what the user expects, but there are far too many services that deserve integration. The challenge Apple faces (with iOS 8, I think) is to find a way to offer this same level of integration, but allow easy implementation with any service the user chooses.