Last week Dave Winer proposed the idea of reading lists for RSS, which are more-or-less OPML subscriptions. I like this idea - a lot - and in fact a few FeedDemon customers have requested this feature in the past.
In a nutshell, the idea is that you'd subscribe to an OPML document which contains a list of feeds that someone is reading, some organization is recommending, or some service has generated (such as "Top 100" list). Changes to the source OPML document would be synchronized, so that you're automatically subscribed to feeds added to the reading list. Likewise, you'd be unsubscribed from feeds removed from the original OPML.
There are a number of implementation details that would need to be worked out (ex: would a FeedDemon user really want to be automatically unsubscribed from feeds dropped from the source OPML, especially if that user had flagged some posts in those feeds for future reference?), but details aside, I'm curious whether this is something you'd like to see, and if so, how do you think the idea can be improved upon?
Just to toss some ideas out, I'd like to see some sort of rating/ranking/popularity attribute added to each feed in the OPML. For example, if an OPML file is generated by an online aggregator, each feed's popularity among the service's users (or a subset of users that you trust) would be included in the OPML. Also, the ability to include a space-separated list of tags for each feed might be useful - that way you could say "automatically subscribe me to feeds with the 'sports' tag," etc.