Great read, and I'm expecting this to happen sooner too.
Think it something like -- the core-emailing is there within gmail, but extend the functionalities via services. Some external app uses to build services on top of emailing.
Something like -- Hootsuite on top of twitter, Hootsuite manages and adds value by its own apps.
Think: we have Gmail powered apps? A link on top like: my-installed-apps -- to easily navigate and find newer apps for your workflows.