Through its MFi program, Apple has released a preview of a connector meant to be used with headphone cables.
Update: According to The Verge, Apple is simply repurposing an older port technology in an effort to help Lightning and USB-C work better together. The UAC will apparently allow you to swap between using Lightning and USB-C cables with the same pair of headphones, with the UAC acting as an intermediary.
Apple is apparently planning to add a new type of connector to its Made for iPhone (MFi) program. Recently launching in developer preview, the Ultra Accessory Connector (UAC) is an eight-pin connector that is smaller than both USB-C and Lightning ports.
The UAC is evidently currently meant to be used with headphone cables. From 9to5Mac:
Apple's specs for the Ultra Accessory Connector through the MFi program currently specify use of the UAC connector (both male and female) for cables used on headphones. Apple will allow accessory manufacturers to make Lightning to UAC, USB-A to UAC, and 3.5mm headset jacks to UAC, which would allow headphones with a UAC port to connect to various Apple devices.
Apple currently utilizes a mixture of both USB and Lightning ports on its own accessories, such as Beats headphones, while microUSB is popular with third-party accessory makers. It's unclear at this time whether the UAC would find its way onto accessories other than headphones.