Another good place to look is the Firefox OS Module Owners list.
The area which is definitely smartphone-specific is the Radio Interface Layer. I would suggest the Contacts API is possibly smartphone specific too (can we do without it and just have the contacts app use IndexedDB?).
- Radio Interface Layer
- Telephony - dom/telephony
- Mobile Message - dom/mobilemessage
- Mobile Connection - dom/mobileconnection
- ICC - dom/icc
- Voicemail - dom/voicemail
- Cell Broadcast - dom/cellbroadcast
There are many APIs being removed as part of the transition project which will therefore not need owners including:
- Web Activities
- System Messages
- Inter-app Communication
- Data Store
- Engineering Mode
- Mobile ID
There are other APIs which are used by smartphones but may still be needed for other Mozilla products like Smart TVs and other connected devices:
- FM Radio
- Device Storage
- Web Speech
- System XHR
- TCP Socket
I would suggest that for each of these APIs we should consider whether they are still needed, whether new web standards can replace them or whether they could live outside of Gecko (e.g. as a local web service).
For example, could Device APIs like Bluetooth, WiFi, NFC, FM Radio, Camera, Device Storage and Telephony be replaced with local web services using HTTP or WebSockets and talking directly to the Android HAL underneath? Are networking APIs like SystemXHR and TCPSocket needed any more if the system UI is chrome and there are no privileged apps? Can we think of an alternative for the Alarm API based on Service Workers? Could we expose Bluetooth and Web Speech to the open web based on the emerging W3C specifications?
The Browser API is now used inside Firefox and is likely to continue to be maintained by Mozilla.
Also remember they are not APIs but there are many apps in Gaia which may require new owners.
So I’d say the most obvious pieces which may need new owners are the telephony APIs and the non-system Gaia apps. For other areas it is currently less clear.