Persistent multiqueue, for parallelism between queues, and serial..ism within them. For example, you might have 100 people to whom you're sending messages concurrently (you're kind of a nag), but within each per-person queue, you want to send in series (so they don't find out you're kind of a nag).
See example for a basic lesson in nagging
The queues are self-ordering, provided you specify the position of each item.
See example-order for a basic lesson in unscrambling eggs
Improve accounting of queues, maybe using level-updown to atomically save a queue name with the append to changes-feed