Luminary has already failed at making premium podcasts for one reason. They make the app. In order to sell their podcasts, they need to make an app with free podcasts as well as their own. It's better to mix 5 exclusive podcasts with 1,000 free podcasts from other platforms than make an app that only has 5 podcasts.
My idea is for a service that you subscribe to, and you are able to authorize one IP address. Then, you open the podcast app of your choice on the device who's IP address you authorized, which is then able to download an episode of that podcast. Otherwise, the server would see that your IP address is not associated with a paid account, and would only download a 30 second preview with a message at the end instructing you to subscribe at a website.
The service could even charge per episode, and charge more for additional devices.