It is an interesting thought. I think it's hard for me to imagine the benefits of moving on from XML overcoming the pain of supporting both formats for literally everyone involved (producers and apps), and RSS is, despite its shortcomings, quite mature. If you are proposing hanging additional data off of RSS with a link but continuing to fetch the RSS for basic episode data, I think it is self evident why that is fraught. And if you are suggesting having an alternative representation, I guess I worry about reliving the pain of the early days, with lots of format fighting and incomplete / incorrect implementations.
How do you see this working?