Appointment.reasonReference should point to request

Appointments are usually created before procedures, so they would normally not point to a procedure. They should, and that seems missing, be able to point to requests, e.g. 

• Appointment in hospital / clinic to take requested meds (point to medication request )
• Appointment in hospital for planned surgery (serviceRequest)
• Go to clinic to get a holter device (deviceRequest)

Not sure pointing to a procedure is within the 80% most common uses, but I would not exclude that, just add the requests.

Discussion: https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Appointment.2EreasonReference