If you're working on a podcast client here are two suggestions for improving your RSS and OPML support.
1. RSS has a cloud element that makes it possible for a feed subscriber to request notifications of updates so that user sees the new stuff more quickly, and doesn't have to wait for a periodic scan. My River2 aggregator supports this as do all WordPress sites. It's a feature of RSS 2.0, and dates back over ten years.
2. Allow users to subscribe to OPML subscription lists. When you read one of these during your periodic scan, subscribe to any feeds it contains that are new to you, and unsub from any feeds that were present in prior scans that are no longer there. This allows a lot of powerful connections. Notably:
Users can link your tool to a universal subscription app, that may get to places yours doesn't. Anything we can do to make subscription easier makes podcasting more useful to more people.
Users can subscribe to "stations" that are curated by experts. Someone who loves sports might put together a set of feeds that cover the NBA right now (because the draft is on) but would remove those feeds and add ones for hot baseball teams as the major leagues head to the All-Star break. It allows people with expertise to provide it in a nice package for users who want help finding interesting new stuff to listen to.
Note that neither suggestion calls for changes in RSS or OPML, and btw, they are equally applicable for general RSS aggreators and readers. But podcast clients are on people's minds currently.
If you have comments or questions post them here.