The idea is simple: besides importing feeds from an OPML, let the user subscribe to an OPML that has a permanent URL. The feed reader automatically subscribes/unsubscribes to each RSS feed found in the OPML. The OPML is periodically refreshed, and RSS subscriptions updated.
tt-rss already has a related feature: public OPML.
There are several use cases that could make sense. From the original post:
It means the list can be maintained by another piece of software, or by another human being. A domain expert, for example. (...) Spotify could give you a list of feeds to follow for your favorite bands, and that list would change as your musical tastes evolve. (...) I want to provide a curated feed list for people who read my blog.
Speaking for myself: I would have done this in my last job, where we had three different people (and wanted to get more) reading feeds and getting news for everybody. I could have easily kept a master feed list.
Another idea: you could make some quick and cheap social stuff. For example, have an OPML for the Published feed of all the users in a tt-rss instance. When there is a new user, the OPML maintainer updates it, everyone else gets subscribed to that user. This could be productive in an organization.
It's probably a niche feature, I think it could make for a nice plugin if someone is interested.