Simple Reminder item
Currently, if one just wants a reminder to pop up on one's screen, one has to create a Task or Appointment and set a Reminder on it. Task is pretty complicated for this and not even really straightforward as the reminder is keyed off of the start date of the task even if the start date is empty and only has a due date.
So while Appointment is more straightforward it still has a concept of a "window" of time over which the appointment occurs and so having to set and end time (or simply ignore it, but look busy in one's calendar for that window) is still a bit awkward.
A much simpler Reminder item would be useful which would simply achieve the goal of issuing a Reminder (i.e. using the existing Reminder options for Tasks, Appointments and Meetings) which could then be dismissed or snoozed using the existing mechanisms.
It could quite literally be an Appointment that doesn't have an end time and therefore doesn't span a time window. I'm not sure if it's appropriate to put it into a calendar though. That said, "Hey Google, remind me to ... at 9AM" does show up in my calendar.google.com calendar. Although I don't see it my evolution view of that calendar so I'm not sure what kind of CalDAV object (if any) they are using for it.