Now that the specifications of Fairphone 2 have been announced, it looks like something along the line might be possible. There are connectors that could be used for stuff like NFC, wireless charging, and possibly also for a keyboard. Somewhere I’ve read the connectors speak USB, which sounds like anything should be possible 
Edit: Giving sources is always a good idea:
Blog post on fairphone.com: The architecture of the Fairphone 2
First, we included an expansion port in the back of the transceiver. This expansion port gives us the option to build alternative back covers with integrated additional functionality – for example, adding NFC for payments. The port is designed to connect to additional circuitry in the specially designed outer case through a set of spring-loaded connector (pogo) pins. The port provides power sourcing and charging and also data connectivity through standard USB signaling.
Emphasis is mine.