Podcasts within podcasts. Audiobooks within audiobooks.
Imagine listening to your favorite podcast or audiobook like normal but with quick breaks for counter-arguments or additional thoughts from a trusted commentator of your choosing. You could switch this audio commentary on and off. Commentated non-fiction audiobooks would be like listening with a really smart friend who pauses it now and again to give some thoughts, point out strawman theories or expand on points. Boring news podcasts could become funny if comedians gave their two cents along the way. Now your most hated audio program can also be your most loved.
This could be thought of as a new form of podcasting called meta-casting.
How it would work
There be a centralized server that would host the commentaries. Meta-casters would create a profile / channel for themselves. They could record their commentaries right into the app, which would then be available in Timeline format for any podcast player app to import and play.