Simple integration to other O365 objects
There are a number of suggestions here that talk about particular elements of O365. I would make it both simpler and more general - I would expect to be able to link directly to an object anywhere else in O365. That may be an email, a calendar event, a document in SharePoint or OneDrive, a note or notebook from OneNote, etc.
The Planner tool is powerful, but at the moment it is essentially stand alone. Even typing links in doesn't really work - they render as plain text and are not active as links. The power of it will come from hooking together all this data.