I would love the option to create dynamic audience segments that are based on a couple of key metrics, instead of needing to create a new audience segment for my content every month.
One common use case I could see is wanting to restrict historical access to content. Currently, I have to either create audience cohorts (with appropriate tags/segments) or manually set a segment based on their registration date. What I propose would allow for me to declare a dynamic segment that would allow only those who have registered before the date the content was loaded... or the date of the first video of a playlist... or some other date metric.
Another use case could be unlocking a piece of content once any other of a set (or a certain number) had been watched.