If you're interested in something that doesn't even exist, and should be more-or-less straightforward:
Music/podcast app that will accept VST plugins (there are many FOSS ones, as well as non-FOSS ones) so that we can compress/limit the sound range on podcasts while in the car. Or even a built-in compressor/limiter that's based on FOSS compressors.
I was listening to a hysterical podcast episode between three people, but one of mics was way louder than the other two. I had to take it into Pro Tools and fix it myself before listening to it.
There are apps that allow EQ, but none that do actual compression, from what I can tell.
The ability to automatically detect commercials (via sound level / machine learning) and skip them would be amazing as well. There's an app for iOS that does this, but nothing for Android.