RSS for Subscribing to Anything

Nick Bradbury writes about "reading list subscriptions", basically subscribing to OPML files. A great idea. But why stop there? Couldn’t we just subscribe to anything on a standardized XML format, by inserting into an RSS feed items conforming to a specific XML schema? I’m most certainly overlooking some technical complications here, in particular, that we would stop keeping things simple. But anyway, by inserting more and more stuff into RSS feeds (we’d like to subscribe to anything, wouldn’t we?) we will finally reach a point where such a generalization is inevitable, I believe.