I imagine it won't happen because in some ways it's kind of backwards, but I hate running with my phone, especially because I have a huge 6+.
It seems like a simple thing to just want to be able to take some of my music/podcasts with me running, but it's not realistic unless I own a separate iPod shuffle that I then have to deal with. Adding a small amount of memory to AirPods would make this possible and relatively seamless.