It seems like many people would like some sort of user-submitted Q&A format. In my experience, the Q&A segments of podcast episodes tend to be maybe 1/4 as interesting as the rest of the episode. People being interviewed are more likely to say broadly interesting things when given open space to ideate with the occasional gentle nudge from the interviewer. It's true that some user-submitted questions will be great, but natural conversation with interesting and intelligent people is by far my favorite kind of podcast to listen to.

A couple examples of excellent podcasts of this variety are the Ezra Klein Show [1] and Sam Harris's Waking Up [2].

[1] http://www.vox.com/ezra-klein-show-podcast [2] https://www.samharris.org/podcast