I feel it would be very useful to be able to create joins between FeatureServices in AGOL. Below is an example use case using AGOL for incident management during a marathon of simulate event.
Publish a table of data containing athletes race number (unique), emergency contact information and pre existing medical conditions including allergies to medications.
During the event there's an incident, using app the emergency responder marks the incident with a point, then enters the athletes race number, which then populates the stored information about that athlete.
There's more to the incident workflow but core is the ability to relate or join data.