This is more than an API, it's a feature for windows 8 and windows phone 8.
An equivalent to airdrop so all those users using your platforms could send any file easily. It should work over wifi ad-hoc (wifi direct), Bluetooth Ethernet zigbee or other.
Just imagine all all those apps on all devices could have a way to communicate with each other.
You could use this to make the win phone office remote app connect to the laptop without the need of a wifi hotspot.
I'm don't think this is a missing API, but rather an API request. FYI, I believe you may already today communicate freely via wifi, bluetooth, and NFC, so I don't personally see a reason for this.
You could also implement the ability to respond to sms and calls you get on Windows Phone from Windows 8 or Xbox like I posted here:
And because this has to be about missing APIs, the API would let apps from one device connect with apps from other ( much like proximity for win 8) but with the support for all Microsoft platforms: win phone 8, win 8 and xbox would be great.
Also ... this could enable us to test from visual studio the windows phone apps remote without a cable connexion.
And make it a mesh network.
It would be great if Apple allows it, to make it compatible with the Apple's AirDrop