Extremely good call on the tagging. I love that. I've implementing saving for some basic essentially bookmarking of episodes so far but taking that one further and allowing the user to define custom save tags that they can associate without episodes and quickly access is a fantastic idea.
I'll have to give your backlog queuing idea some thought. I think there's absolutely room for that feature. There's also a lot to considering with implementing it in the most effective way possible. I wondering about maybe taking a look at what podcasts you listen to the most, checking which episodes from those you haven't listened to, and then aggregating them into a cohesive backlog. These kinds of computational tasks like refreshing for example already happen on the backend so adding more things like that doesn't affect the user experience at all.